Back when that rifle was new, it was common the press or clamp on adapter to thread the suppressor on. Much more common on Rimfire chambered guns. The problem with that was fitting the adapter over the existing barrel just about guaranteed that the front sight would be useless. So turning the barrel down to have the adapter fit the barrel profile makes sense. It’s possible that the suppressor itself was a clamp on.

The video uses an interesting rifle for the example

(You need to Login or Register to view media files and links)

Article has a complete Maxim catalog
(You need to Login or Register to view media files and links)