aparently the OP's Sears and Roebuck Muzzleloader is indeed a Miroku.It's looks just like the Miroku Kentucky rifle pictured in this thread:
value of Miroku Japan/Sears .45 kentucky rifle
http://www.thehighroad.org/showpost.php?p=5075315&postcount=1
sarg said:Looking for current value and powder charge for the sears Roebuck Kentucky Rifle (45 cal). want to shoot Pyrodex"P"
rocklock Post #10 said:Although they say .45 cal, my thinking is, it is closer to .44. The reason is, I can barely get a .440 ball down the barrel with a .010 patch, but a .433 pistol ball and the same patch work great.
http://www.thehighroad.org/showthread.php?t=264672&highlight=ultra