Hey Nanomat. You seem pretty knowledgable on recognizing firearm parts. Do you have any idea what this leaked pre-release MAB 38 variant looks like? It seems to have the barrel perforations of the release "Orso" and a muzzle device of some kind. The muzzle device seems familiar but I can't quite remember what it was.
The pic you linked doesn't work anymore but I suppose you are talking about these things here. And yeah AugFC is right about the MG34 muzzle but I don't agree with the MP5 mag. I see where Aug is going with the little groove going along the mag but I see two issues. The MP5 mag is too straight while the depiction is more curved and that "groove" is only present on the icon and might even not be a groove at all. The actual in game model (which is the extended mag) is actually the Type 100 mag. See here and here.
The videos you linked are showcasing the extended magazine model, which is a seperate thing. The magazine in question is the default magazine for a weapon variant that was cut from the game, so there is no in-game model that we can examine, unfortunately.
The groove and the latch at the top are definitely lifted from the MP5, but you're right about it being too curved, so I'm guessing the developers just re-textured the T100's magazine to make it look like an MP5 mag.
These factors considered, I think it's fair to say it's not an 100% match for any specific magazine.