Whatever you do, do it properly, there is nothing worse than a chewed up screw head on a gun that was previously untouched (if this is the case).
Other than that, it is probably pre-'67 approximately as IIRC this was the year when dates started to be stamped on the cylinder. If it has a serial number, under the barrel close to the underlever, which underlever is folded metal rather than machined from solid, it is not much older. Older ones had no number.
How much older, only pics will be able to enlighten. There were many versions of the model 50.
Feel free to send me a pic to my email address.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS