The "COLT's" is right up at the front of the cylinder and it doesn't take a lot of rust or corrosion from firing to obliterate the "C" and sometimes the "O" or part of it. The whole legend was put on by rolling the cylinder on a hard steel plate with the scene in reverse, so it is not very deep, unlike the (last four) serial numbers which were put in with single number stamps and a hammer.
Jim
Jim