I have seen this type of key and lock usually on industrial equipment and I'm wondering if a spare can be cut from a blank. In this case it is on the gas bottle locker of my caravan and I only have one key for it.
The part of the key which fits into the lock is tubular, with protrusions inside the tube to match the lock. It is marked 'Southco' and also bears a number. The lock itself has a matching circular slot, the centre part of which is turned to lock and unlock.