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
I suggested it in the past
Make Luftfaust available to rifle men. It was never made to destroy tanks so there’s no real reason why only AT men get it.
This is essentially a rifle grenade, not a mortar, so it wouldn’t be a RMN-50 equivalent
This for example could fit that role, however there is no suggestion about it yet
It’s the other way around:
Blanch-Chevallier and others launch simple rifle grenades with same HE load as a Mills bomb (and HEAT rifle grenades).
What OP describes is an American way to send proper 60mm mortar shells from a rifle base, no need to replace the barrel or anything else.
With proper manual and all
Heavy is the burden of being me… 
I’ll add it to the list of suggestions I’ll make, Rider II suggestion just dropped. Had to play a few games again to even post again.
More to come, soon TM.
no need make rider suggestion, look my posts <3
APC and Rider Ticket Rework ticket cost suggestion
Rider Class Improve updated biker squad
Possible Biker Rework Concept original biker squad update
We have different visions, you may consult my suggestion on the matter.
At any rate, it is already out and over half an hour old as of me writing this reply.
I am now intriged
