No date stamp = pre 67.
The stock is 50B
The rearsight, that's odd. I never saw that style of rearsight on a 50B.
Normally found on slightly later years possibly 67 on when they started stamping the date.
If after removing the rearsight you find proper threaded inserts to accept it's screws then it's a factory job if you find just threads through the cylinder then someone has fitted the rear sight.
However it does look very correct. The presence of a full length scope rail also indicates that this is probably a "late" 50B when it had the older style of rearsight the rear rail would normally be only just long enough to accept the rear sight with diopter fitting.
So does the front sight match with it being relatively late. I'd say somewhere close to '66.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS