Hi Joe......welcome to TFF.
I'm not really sure what you've got. There is
a Cimarron Gun Works in Farwell, MI, but they're a repair place and don't make any guns: http://www.jayssportinggoods.com/gun...ricingList.htm
And, of course, there's Cimarron Arms, in Fredricksburg, TX, that sells Western-type reproductions (mostly Uberti & Pedersoli): http://www.cimarron-firearms.com/
Cimarron Arms does
sell a number of black powder pistols, but none in single shot.
There was, or is, a Frank DeHass who wrote a number of books on gunsmithing and building single-shot guns: http://www.brownells.com/aspx/NS/sto...aspx?p=5737&s=
.....which leads me to believe that your uncle's gun was custom built, possibly by Frank DeHass or somebody associated with him.....or perhaps by your uncle himself from plans & parts supplied by DeHass.
Hope this helps you figure out what you've got.