It is a 3/6 "T" handled Allen wrench, a hex key, I don't know how old it is but I havn't seen one before. Maybe in England they are called something else. In US we call them allen screws and allen wrenches.
Ah, got it, thanks Larry, I couldn't see in the pictures. Nice old wrench. We call them Allen screws here too, and usually use the 'L' shaped hex keys to turn them but T handle ones are available.
They have an interesting history, introduced to save injury in the days of belt driven machinery in factories and line shafting, the recessed screws prevented injury from getting caught on the heads of square locking screws. It seems they faced opposition from people who didn't like the new method, a bit like torx fasteners do today.. https://en.wikipedia.org/wiki/Hex_key