‘And this has gone on as described. I performed the fourth regular transaction at the beginning of April.’

‘The regular transaction?’ asked Joe.

Robertson paused. ‘There was a further one, out of pattern, you might say.’

‘And can you say precisely when this one occurred?’

‘Yes.’

He selected another leaf from the file and handed it over. ‘You will see that the value varies. This one mentions the sum of three thousand rupees. And it is dated 1st May 1921. It was shortly after Mrs Sharpe’s brother was killed. I remember she was wearing black and she chose a diamond and jet mourning piece.’

Joe looked at him closely. There was no hint of suspicion or suggestion in the bland, dark eyes.

‘Who collects the contents of the blue box, Robertson?’

‘No one I know. It’s a different messenger each time. An Indian. I suspect just someone picked up in the bazaar and given this task for a few annas. I have no doubt the messenger is carefully watched, of course, but as to the identity of the watcher or indeed the destination of the blue box, I have no idea. My responsibility ends when the box leaves here.’

‘Have you a feeling about all this?’ asked Charlie. ‘Share your thoughts with us. You must have formed some kind of theory about the exchange. Embezzlement? Extortion? Blackmail? Generous donations to an anonymous recipient?’

Robertson’s eyes gleamed for a moment. ‘Probably two out of the four,’ he said and appeared to be unwilling to take the thought further. ‘You may be interested,’ he went on after a slight pause, ‘in seeing this. It was put through my door this morning.’

He handed Carter an envelope. With an exclamation of dismay, Carter took it carefully by the edges.

‘I shouldn’t worry about obliterating any useful fingerprints,’ said Joe. ‘The world and his wife will have handled it by now – everyone, I would expect, apart from our, er, customer. He’s not going to make the mistake of leaving prints on it. Go ahead. Open it.’

‘Let’s see. “Mrs S. will buy more jewels. Value five thousand rupees. Same arrangements.” Mmm… price has increased significantly. I take it Mrs Sharpe hasn’t appeared yet?’

‘Oh yes, she has. She came in very early – about half an hour before your good selves. She chose a diamond solitaire ring and she gave me a banker’s draft in payment. And I have completed my arrangements in regard to the second part of the transaction.’

‘Would you show us the routine with the blue box then, if you’ve prepared it?’

They went back into the shop and Robertson took a small velvet box from a drawer underneath the counter. They peered inside. Coiled in the bottom and glittering even in the half light was a diamond necklace.

‘Very simple. Practically unrecognizable. Easy to break up and sell as individual stones,’ Carter commented.

‘Look,’ said Joe, ‘Robertson, would you have any objection to varying the routine a little? We desperately need to know – as I’m sure you’ve guessed – the identity of the person who is the recipient of the contents of the box. Mrs Sharpe’s peace of mind, to put it simply, is at stake.’

Robertson nodded his agreement.

‘What I want you to do is change these diamonds for something a lot more distinctive. Something so unique and decorative that wherever it appears again – if ever it does resurface – any jeweller would recognize it.’

‘I see,’ said Robertson. ‘And then, delivery safely accomplished, the Simla police circulate a description of a certain piece of stolen jewellery so unmistakable that it cannot safely be worn or sold without word getting back?’

‘Exactly,’ said Joe.

‘What if he objects?’ asked Carter. ‘Of course,’ he added, answering his own question, ‘then he contacts Robertson again and perhaps in his anger gives away more than he meant to? At least we’d have another handle on this discreet charmer. Come on – what have you got to show us, Robertson?’

Robertson hesitated then with a conspiratorial smile went into the back room and emerged a few minutes later. ‘I think you would agree that this fulfils your requirements,’ he said.

Joe and Carter looked and gasped.

‘It’s perfectly lovely,’ said Carter, ‘but it won’t do! Nothing approaching the value you’re supposed to supply. I mean – it’s… it’s… what do the ladies call something like this? – costume jewellery, yes, costume jewellery.’

‘No it’s not,’ said Joe. ‘I’m sure I’ve seen something like this before… on a portrait perhaps?’

Robertson smiled and nodded. ‘You have it. On a portrait by Hans Holbein. Sixteenth-century German portrait painter. The Tudor royal family were much painted by him. They liked to be seen wearing rather spectacular jewels, like this one.’

They looked again. The whole arrangement was perhaps four inches across and five inches long. At its centre glowed a stone which could have been a ruby, Joe thought, had it not been so large. It was surrounded by a gold circlet inlaid with bright enamels in the form of Tudor roses and posies of glittering clear stones which Joe would have sworn were diamonds.

‘The style became very popular again in Europe some years ago and these pieces began to be produced with showy semiprecious stones like peridots at the centre. They’re called “Holbeinesque” in the trade.’ He paused for a moment, looking at the brooch in rapt admiration. ‘But there’s nothing “-esque” about this one. This is a genuine sixteenth-century item. Any jeweller would recognize it if it passed through his hands. It’s the Duke of Clarence Ruby.’

‘If that’s a ruby, isn’t it a little over the mark?’ Carter wanted to know.

‘Yes. Far over. But in the interests of saving Mrs Sharpe even a minute’s concern, I’m sure it is worth the sacrifice,’ he said with his deprecatory smile. ‘And besides I did pick it up as rather a bargain. It was the property of a prince. He bought it in London and gave it to his senior wife. She was not grateful. She hated it. Couldn’t see the point of it and came and ordered me to swap it for a gold necklace she’d seen and matching ear-rings. I was happy to do so. Buy, sell or exchange, I get my commission, you know. But of course, if I were ever to sell it on the open market and it were to appear on the bosom of – let’s say the Vicereine – there might be problems.’

‘I see,’ said Carter. ‘In that case, it’s perfect. Any means we have of flushing out Mrs Sharpe’s unknown correspondent must be made use of, Robertson. I’m sure you understand. We’re grateful for your co-operation and, look here, one more thing you can do to help – it’s just a small thing – we’ve got the shop under discreet surveillance. When a messenger comes in for the box could you alert my men? Give them some sort of a signal?’

Robertson smiled and nodded compliance. Joe had little doubt that he was aware of Carter’s discreet surveillance. ‘Of course, Superintendent. Nothing simpler. The window lights are normally switched on. When the messenger asks for the box I will switch them off. The switch is here to hand under the counter.’

‘That will do well,’ said Carter.

With mutual assurances of esteem, they left Robertson and went out into the street. Blinking in the sharp sunlight, Joe screwed up his eyes and surreptitiously glanced up and down the Mall in an attempt to locate Carter’s surveillance team. He saw the usual bustle of European shoppers, Indian servants and street urchins. Two nursemaids walked by chattering and scolding. A Hindu holy man sat patiently opposite, cross-legged, with his begging bowl in front of him. Hesitating on the pavement’s edge, Carter waved away two rickshaws competing for their custom. Avoiding them, he stepped off the pavement into a puddle left over from a late night shower.

‘Drat!’ he exclaimed, running a fastidious eye over the spatters on his smart boots.

‘You’re in luck,’ said Joe, pointing across the street. ‘Look there!’