This was a very successful lock. It was made to at least 1939, and a variant continued to be made through the 1960’s. It probably started late 19C.
It was made in several versions: single- and double-sided, deadlock and 2-bolt, and a smaller version as a latch. 5″, 6″, and even a few 7″. Normally supplied japanned with steel keys, it could be supplied with bronze keys. Also available with 4 or 2 levers, and a similar mechanism as a mortice lock with 6, 4, or 2 levers.
A flanged version continued through the 1960’s, possibly later, much used on ships.
The later ones were lacqeured not japanned.
An old established locksmith I knew in the 70’s had several in stock, but they were very slow sellers by then. They were always expensive.
With large sheets of mild steel much more abundant and cheaper by the last quarter of the 19C, builders of upmarket suburban houses sometimes included a secure (sometimes walk-in) cupboard of 9″ engineering brick, with a single-plate door and one or two deadlocks. These Chubb ones were popular choices. I’ve not seen any post WW2.
The big heavy rimlock (as well as expensive), was fast losing market share with the lighter, often glazed, doors of the 1920’s- on. I have a 2-sided deadlock also japanned, of 1929, and a flanged one of 1965. Although there is provision for a detector in this, by then it was an optional extra.
Brass single-sided locks with bronze key would be a good choice for an armoury, and there was still much ammunition used that was loaded with black powder. That heavy stamp on the bolt is, certainly, a broad arrow. So the OP’s lock was used by the army, but the model was used much more widely.
Why a rust-proofing treatment was used on a brass lock, stumps me; but that was the standard finish. Probably cheaper than polishing, both for the factory and end-user.
