‘Exactly,’ continued Holmes. ‘And that would explain why the box is so heavy for its size. One cannot drill into it to unlock it, Watson. Not easily, and perhaps not at all. I wonder where your family might have come upon such a box?’

‘It was commissioned,’ said Lossop. ‘I recognize the workmanship, and I know the maker. Or I think I do.’

‘Perhaps this person could help us?’ asked Holmes.

‘No. He is dead some five years. I have seen his work once before. My mentor, Andelan Schutz, showed one of his creations to me. He could not unlock it, but I did. It took me a month, however.’

‘A month!’ I exclaimed.

‘Excellent that you exceeded your mentor,’ said Holmes.

‘Is that not the purpose of having a mentor?’ murmured the man, taking the box back into his hands. Holmes’s compliment had landed well. ‘I relish the challenge.’

‘Then you will open it for us?’ I asked.

He set down the box abruptly. ‘Perhaps. If you can meet my price.’

‘We are not wealthy men,’ said Holmes.

‘I know that. You, at least, dress well, if conservatively, but you have a hungry look. You—’ he turned to me, ‘are more in fashion, but lack great means.’

Holmes had removed his chequebook.

‘Put that away,’ said the locksmith sharply.

‘Yes, do. I will pay for this,’ I said, reaching for my own, but then remembered that Holmes had locked it back in a drawer, having moved it from the one I had smashed open. That he thought to bring his own but not give me back mine gave me a moment of pique.

The locksmith held up a hand. ‘I have not yet named my price. And I doubt you will have what I require, Doctor.’

Before I could rise to the insult, Holmes replied, ‘What is your price, Mr Lossop?’ His tone made me uneasy.

Lossop smiled and stepped back from the counter. Turning to the wall behind him, on which were shelves and shelves of locks, bolts, keys, tools and parts, he pulled a key which he wore on a chain around his neck and leaned towards a keyhole under a small handle at the back, dead centre of these shelves. There was the sound of a click, and to my surprise the shelves split and rolled apart to reveal a secret wall behind them.

On this wall were hung a series of locked metal boxes of various sizes. Each had a tag hanging from it, but the tags faced the wall, and I could see from my angle that something had been written on them, but what?

‘I require a personal commitment from you. And a personal token. Something which you have in the past taken great care not to lose. Something that is very dear to you.’

Holmes was stone faced. Had he known? I, on the other hand, was puzzled in the extreme.