I could not find out much about this Old Timer. Except that they are very rare and use to be fairly accurate. As they were mostly used in use to be very popular shooting contests.
Obviously it was a Percussion Cap rifle.
I am thinking in the 30 caliber range from the photos. Grumpy
FYI – This is a “dual Ignition” version. Very common in these models. The firing pin is missing. The plan was to shoot the rim fire case, then when finished, save the case and punch a small hole in the center of the base. The case is refilled with black powder and a bullet. At this point, a percussion cap can be put on the nipple and it will ignite the powder in the case. I’ve done this with a couple of mine and it works surprisingly well. Bruce