This:
So maybe he's being set up not as sixth-floor gunman but as having owned & supplied rifle used.
Connection to Commie => Commie connection => boom!
If the conspirators wanted to frame him as the Lone Nut sixth-floor shooter, they went about it in a weirdly underwhelming way........
And if, as you believe, a Mauser was found on the sixth floor, how would this be part of a plot to frame Mr Oswald as the Lone Nut sixth-floor shooter?