In WW2 there was never a "Mount B", that is a name only for this game.

MG34 was designed to be a universally used machine gun.
The standard MG34 was used by infantry.
But it was also used externally on vehicles like Sd.Kfz. 251.
And the MG34 was used internally on early tanks like the PzKpfw I.

Brief history of MG34 by Ian McCollum. (published author on various small arms)
Devs may consider contacting Ian who's referenced for other games like BF1.
(Unfortunately I didn't find a video by Ian on MG34 Panzerlauf)
https://www.youtube.com/watch?v=E-KgQ-OZJZ8


In game the fraudlent-"Mount B" shows MG34 Panzerlauf barrel but jacket is wrong.
The real MG34 Panzerlauf was mounted in tanks, used bag and usally had buttstock removed.
In an emergency tank crews could dismount MG34 Panzerlauf, then retrieve a stored buttstock, drum, and bipod.

There was many prototypes like MG34M, and some of them had limited field tested like MG34s, and some even saw very small production numbers with limited use during WW2 like MG34/41, and like the similiar looking MG81Z Zwilling which was mounted in Aircraft in pairs.

MG34 was to be replaced by MG42 which didn't accomplish that during WW2 because of limited production.

Here's an image showing
1. standard common MG34.
2. MG34 Panzerlauf with its unperferated barrel jacket for use inside vehicles.
3-5. couple prototypes, and lastly the MG42.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2600706020


There's websites with info and photos of real MG34 Panzerlauf:
http://www.deactivated-guns.co.uk/index.php/axis-deactivated-guns/genuine-wwii-tanker-mg34-panzerlauf-or-panzermantel-with-extras/prod_2372.html
And its true that these are now a rare variant:
https://sunji53.tumblr.com/post/44597646073/gunrunnerhell-mg34-panzerlauf-these-are-more

Plenty of websites, books, and WW2 photos document this.
https://www.militaryfactory.com/smallarms/detail.php?smallarms_id=63
https://en.wikipedia.org/wiki/MG_34#MG_34_based_improvement_attempts