Do you believe that the rifle that Weitzman and Boone discovered was a 7.65 Mauser? Yes or No.....
There is footage taken on the 6th floor on the day showing Oswalds rifle. So Oswalds rifle was there.
In theory, a police officer could have gone in to the TSBD with an unregistered (illegal) rifle. Put it down while searching the TSBD with the other police officers and one of those police officers then picked up his unregistered rifle and thought he had found the assassins rifle. Rather than admit that the rifle was his, the police offficer who owned the rifle said nothing until later that weekend when he admitted that it was his rifle. Then the DPD covered up the issue from the public. Police officers do stupid things all the time.