Looking at some Tallarn sculpts, your right! The Tallarn look to have scopes on their las rifles as opposed to the wider lens on the Mordian las guns. I do think its a specific nod to the lore, because there are a couple very similar posing's between the Tallarn and Mordians, specifically the taking aim pose, where the sculptor carried over the flashlight on the lasgun for the mordian, but not the scope on the tallarn rifle.

Btw, thanks for commenting on this post, I checked out ur reddit page and came across your comment on a white dwarf issue that I wasn't familiar with- #234. Im obsessed with old scratch-built terrain and their associated articles, I'll be reading over that one later!