I'm not expert on trigger guards either, but my swag is it's a pistol trigger guard. Enfield made a muzzle loading pistol that the screw hole is in about the same location as your
find, and the downward bend at the heavy portion of the brass makes me think pistol. I don't think your finds screw hold lines up for the enfield rifle, but if you put a couple of dollars
with those ideas, you can get a cup of coffee at Starbucks.