Clearly, as with most makers, there were some “transitional” models with a mix of old and new features.
My guess (guess) would be that Diana decided to introduce the metal sight with the new stock, but still used up their inventory of earlier stocks with it for a little while. The plastic sight was still being used into the 80s on the 5 and 6 pistols and the 25 rifle, so there would have been no imperative to keep using it on the 50.
I’ve only had one 50, a post-76 square stock model and TBH I’ve not been that impressed with it. On the other hand, 70s Airsporters are pretty unimpressive too, and the Webley Mark III was out of production in 75, so there wasn’t much competition.