The Mk2 or ll on the backsight leaf indicates that the LEAF is the Mk2 type, as in 'LEAF, Backsight Mk2' but the actual sight as a whole is a BACKSIGHT, Assembly, Mk3. Don't ask, just one of the vagaries of the UK Military VAOS nomenclature system. I started to explain this in the 'found some Enfield parts' thread.
With regards to the stores system, most Soldiers didn't worry toooo much about all the nomenclature crap. Their sole aim in life was to bamboozle the storemen (or blanket stackers as they were irreverently known....) and con them out of or just plain thieve as much as they could - by fair means or foul