Guess this is a stumper. Only hint I've gotten was Marlin must've done some for a hardware outfit called "Simmons" at some point. They were in the Nebraska area for sure. this is a 1947 vintage gun, and other than the "odd" butt plate, looks stock Marlin for the era.
I was hoping someone could confirm or debunk the info......Buck : ???
I would guess the original Marlin buttplate was broken, and it was replaced with a Simmons shotgun buttplate of the same era. Times were tight, and that was available!? see gunbroker auction 222122019 for an example...
Definitely not a stock or special item in my opinion, and experience, with years of collecting the model 39/39A.