This is substantially interesting. The second picture above is extremely uncommon- Number one, it shows a ground adapted MG-131 13.2mm aircraft flexible gun, a rare-enough picture as this was an uncommonly adapted weapon for ground service ('Erdekampf') perhaps being only slightly less rare in that use than the MG-151. Second, if you look very closely you will note that the MG-131 shown here is actually fitted with a very common MG-15 third variant airborne flexible gunners sight set. I have extensively researched the MG-131 history of usage and have yet to come across such an adaptation for the common MG-15 sights shown adapted there.
This is actually a significant picture as it potentially adds a new unrecognized adaptation for the ground use MG-131's.
What makes this interesting is that for starters the barrel jacket diameter of the MG-131 is nowhere close to the MG-15 barrel jacket.....causing the field creation of an ersatz adapter bracket. And, even more importantly the sight regulation of the MG-15 ring diopter is completely wrong/useless for the 13.2mm x 64RB cartridges' exterior ballistics curve.
It probably was a heck of a handful to shoot like that.