femurat wrote:The number 515501 is stamped on the lock body, but unfortunately I wasn't able to find UK patents online. Anybody has a link?
Took me a minute... the stamping on the case is a bit unclear: the patent number is actually 515561. The patent was submitted in 1938; you can view/download the patent here:
UK515561The key's an interesting design... and you're absolutely right: the key tip does need to be longer than the hole diameter.

The key shaft is actually a hollow tube; the key "nib" is initially inside the tube while the key is inserted:
When the key is pushed into the lock, it causes the nib to extend out through the slot in the bottom of the key:
And, from there, rotating the key will cause the nib to push against and retract a simple lever-latch mechanism that's holding the shackle in place (the shaded #14 in the diagram):
From a quick read of the patent, it seems to only talk about a single locking lever, so I don't think it's a complicated key. It also looks like the locking latch could also be shimmed fairly easily.
Nice find!