Picked up a No. 4 Mk II last year (1950 dated Fazakerly) and have enjoyed shooting it, but decided to go down the rabbit hole of seeking out one of the original diopter sights made for the rifle in the 50's and 60's.
Took me a look time looking, but finally secured one from a seller in England. It arrived last week and I was able to fit it up quickly to the rifle. Everything mounted using stock holes so no modification was needed to the rifle.
Looks period correct as the sight has almost the same patina as the rifle.
(https://i.ibb.co/bdzWc3K/20210703-131534.jpg) (https://ibb.co/3hBrgsC)
Nifty neato keen. :) I like the Enfield #4, but prefer the caliper sights myself. Still it looks really nice when you can get a part that is OEM equipment. Great find.