During WWII, the Germans had a grenade launcher device on some of the Kar 98ks. The device pictured in the film stills isn't it.
The rifle is identified as a Gew 98, which it a WWI rifle, and they didn't have rifle grenade launchers for those.
based on what I see in the pics, best guess is a dummy item, attached at some point by the prop dept for something or other (or possibly just because it looked "neat" -or what ever the slang was at the time).
In LOTS of the old movies, historical accuracy & period correct firearms were not a priority. Literally what ever was in the prop dept that met the general idea (like bolt action rifle, or semi auto pistol) was what got used.
SOMETIMES, it was something used because it would work with blanks, like SGT York using a Luger in the movie, because it worked with blanks and they didn't have a 1911 that would...
In the old westerns one often sees model 92 Winchesters being used in an 1880s setting. It was a lever action, and so, was "close enough".
The current trend for historical accuracy in both costumes and equipment didn't really start until the 60s, and still isn't universal today...