We had a "1875" category for a few years in Germany.
That was conversions only for pistols, +.40 cal lever gun model built before 1880 (no short strokes allowed in either pistols or lever gun), and hammered shot gun of 16 or 12 ga.
Ammo was either BP or nitro, MIP at least 120 for pistols and rifles.
The category met with considerable interest during the fist few years.
But it died off, unfortunately because eventually there were just not enough cowboys & cowgirls owning conversions because new licenses were/are difficult to come by.
Long Johns Wolf