I have owned several of those guns the first of which I bought NIB in 1980.
I can say positively they were the Williams WGOS ( Williams Guide Open Sight} rear sight. They are still made but they are much more expensive now and not finished the same. The old ones on the Sakos were hard gloss anodized with white lettering and hash marks. They were common OEM sights on many better factory sporters in the 70s and 80s.
The fastest and cheapest way to get one would be on ebay. Lots of take off sights there. The screw spacing is the key ask the sellers for th einfo before you buy.. Should cost no more than $35 bucks
