It looks like you have an interesting scope. It is a hybrid between the No32 Mk2 & Mk3, called the Mk2/1. It is essentially a Mk2 scope with Mk3 Range & Deflection drums fitted. Some existing Mk2 scopes were converted to Mk2/1 configuration after WW2 but your scope was made as a 2/1 buy the United Instrument Company, which percieved wisdom says was a branch of the famous Vickers manufacturing concern. They were produced in modest quantity during 1944, but did not go into larger scale production presumably because of the arrival of the definitive Mk3 scope design in the autumn of 1944, which was cheaper to produce. The OS 1400 refers to the 'Optical Stores' part number, which was unique to each Mark of optical device used by
British
& Dominion forces.
The number on your scope bracket indicates the serial number of the rifle it was originally mated to, & the serial on the scope tin indicates likewise, although they are both unfortunately mis-matched to your rifle. Incidentally, all three rifle numbers that you list would be 1944 BSA produced rifles.
Hope this helps a little.