T20 was closed bolt when it was firing in semi-auto and open bolt in Full-automatic
they changed the open bolt for T20E1 and made both semi-auto and full auto fire from closed bolt Since I quote
“In October of 1944, the rifle had been proofed and function tested at Springfield Armory, and was sent to Aberdeen Proving Ground in Maryland for testing. However, the evaluators soon found that not only did the open bolt feature not significantly improve the gun’s cook-off characteristics, but that it suffered from poor timing. Depending on where the bolt was in its travel when the shooter released the trigger at the end of a burst, the action might come to rest in either the open or closed position. Because of these problems, the open/closed bolt requirement was dropped.”
Now that you mention it if the devs modeled The T20 early rifle to fire from both close and open bolt then changing the name wont work ,either well guess we have to accept that devs took some creative liberties to make the gun have bayonet