Kiwi, that's a good question. My uneducated guess would be that the sights may have been ordered in a batch and the marking was omitted.

RobD, All of the marked ones I've had/seen were 5A for the SMLE. I've had quite a few of the earlier No.9 and similar sights for the Long Lee as well as the fixing plates but don't recall seeing any with ownership marks. You can bet all of the available target sights were used by the South Africans in competition. I've had a few of the rifles you mention with post 1910 BSA barrels too including a CLLE. Some were commercial contract rifles too.