Trying to find a counterpart to give the Americans, I’ve found some photos and a video of a guy talking about this very strange weapon, an improvised mortar made from a damn Garand. I don’t know if it’s what one would expect, but it’s the best I could find so they can have something similar to the RMN-50 or the Arisaka mortar
That seems more like an improvised rifle grenade, but I have heard of an experimental grenade launcher from late war. Im sure it will be resurfaced
like the Arisaka Mortar, and oh well, the weapon you say it’s from the 50s
Just give the Allies captured Arisaka Mortars. The Japanese have so many captured weapons as it is, and it’s highly likely that the US would have captured Japanese weapons as well. So, just give the Arisaka to the Allies, and captured RMN-50’s to the Germans. 
welp, I wouldn’t say no, if germans got the ampulomets because they captured, I could give the chance about the arisaka for allieds too 
Ehem.
A No. 4 Mortar, WW2 prototype:
Blanch-Chevallier grenade launcher, WW1 prototype:
As @GiltBurrito7238-live mentioned, what you have provided looks more like a adapted rifle grenade, meaning that the base rifle underneath would still be useable as a rifle, not at all similar to the RMN-50 or the Arisaka mortar who have one function only.
These two above are plausible weapons which could be used for the Allies, that fit the same niche role as the previously stated in-game examples.
https://www.rifleman.org.uk/Enfield_Rifle_Grenades_and_Dischargers.html
