Question - trying to identify a 1942 No.1 MkIII unusual rear sight

[Son]

Hello, quebecvfr.
The sight on the rifle looks to be a "Premier", made in Australia. IIRC, they were not as predominant as some of the other brands available to range shooters, but look to be of high quality.
I'd call it rare- you don't see many about.
The pics below came from a mate who sold this one a couple of years ago for over $250.