Welcome to the forum , pixeequeen2002 .
Since you do have the lead of the original manufacturer , that's a plus .
Billings was (& still is , somewhat) a center for the mink trade , with roots going back past its' statehood .
Although I was not able to locate much info on the company on-line , I'd guess there will be folks in the Billings area that do know things about the company , perhaps even contacting a wholesale furrier there might just get you answers .
You're not alone in owning a non-PC-component item or two , as most of the folks here do too ; stuff such as ivory , sheep-skin , weapons ( & on & on - the list is really long!!! ) .
Perhaps you could closely inspect the lining of your item to see if it has been tailored to fit your grandma , or if it was not .
It used to be a bit of an occasion , when a woman received a mink .
Maybe someone in your family might know something about that ... like where she lived when she got it , etc. ?
I'd say the best shot at selling might be a vintage shop in a large city ( I see such things around the Seattle area ) , if you can't find a buyer on somewhere like craigslist .