Small Cog in a Large Machine?

I spent 90 minutes today interviewing two very cogent and articulate elderly gentlemen.

They had kindly offered to tell me their stories in connection with a book idea I am researching.

It's a naval history book.

One of the gentlemen, let's call him 'D', had served as a young midshipman on a LCT(R) - Landing Craft Tank (Rocket) - assigned to bombard the Normandy beaches in preparation for the imminent assault landings.

His vessel survived the great storm that followed the D-Day landings only to have her back broken by the explosion of a naval mine. He survived, and later commanded his own LCT(R) in the Far East.

I suppose he too was a small cog in a very large machine. A very brave 'cog' nonetheless.

During our conversation the second gentleman, 'R', showed me this interesting souvenir, composed of more small cogs from a large machine.

He told me what it was but I'm not going to reveal its nature just yet. Anyone care to hazard a guess?