A number of examples of the mount Sarg shows above were sold on eBay out of New Zealand, so they may have been made there, before or after WWII.

For Lee Speeds, I would say most of those that were scoped were done in the 20s and 30s, often using surplus sniper scopes from WWI, and often surplus mounts as well, though only certain types. Considering that thousands of such scopes were sold by the War Department in the 1920s, it is surprising how rarely one sees certain types of mount, such as Purdey or Holland & Holland. Probably the gunmakers preferred to supply new-made mounts.

On the other hand, the persistence with offset mounts despite the lessons of WWI could be interpreted as using up stocks on hand. But then again one can never over-estimate the inherent conservatism of British industry eh?!

The Gibbs is an interesting set up and I have photographs of similar mounts they supplied to the War Department in WWI.