Personally i dont believe this type of knob was used by yugoslavians, they would add a brass one knob, but You are right about the rivets, but this could be a later replacement as normally similar secure strap could be sewed into, like other germans frog got added the secure strap in late 30ies.
I think we are saying nearly the same thing, Andrej. Retention straps were added to standard frogs by the Germans. And they were even added to captured frogs. But not in this manner. As far as the aluminum stud (knob) on the strap it is clearly not Yugoslavian. It is German as stated in my original post. The stud is not brass because it is not Yugoslavian. The entire strap is German but not added by them. The oversized flat-head rivets with hollow backs scream Yugoslavia. I agree with your comment that German retentions straps were sewn-on; not attached with rivets. My thoughts only ....