I'm with Martini-Henry on this one.
Generally, folks shot what they could afford, or scrounge, or steal. In the years following the Civil War money was very hard to come by (particularly in the West and the South). So, as long as the pistol(s) kept on doing the job for them, there'd be little motivation for much more than improvements (such as cartridge conversions). Therefore, I'd expect to see NMA- and Colt-conversions in pretty regular use up until the turn of the century, and in some places maybe even later.
The above is my impression/opinion only. I'm sure others,who possess much more knowledge than I do, will chime in shortly.
I shoot a brace of well-worn NMA's with conversion cylinders, BTW.
Regards,
-- Nighteyes