I think sometimes it's done for legal reasons (rifles below a certain barrel length in the US are special and need some extra permits, but permanently attaching some kind of muzzle device counts as increasing the barrel length, and using a dummy suppressor specifically can get you the aesthetic of suppressed compact carbines like the AKS-74U or CQBR without the legal hurdles), but in this case it might not be that, since the M1 Carbine already has a sufficiently long barrel. The page I got it from states it might be a movie prop or display item.
What's the dummy suppressor for?
I think sometimes it's done for legal reasons (rifles below a certain barrel length in the US are special and need some extra permits, but permanently attaching some kind of muzzle device counts as increasing the barrel length, and using a dummy suppressor specifically can get you the aesthetic of suppressed compact carbines like the AKS-74U or CQBR without the legal hurdles), but in this case it might not be that, since the M1 Carbine already has a sufficiently long barrel. The page I got it from states it might be a movie prop or display item.
https://www.wargm.org/armoury/index.php?mode=2&cat=Fully+Automatic+and+Self+Loading+Rifles+and+Carbines&item=125