Dennis Adler in his book “Colt Single Action From Patersons to Peacemakers”, mentions that 60 long cylinders are known. He says, according to McDowell there are several schools of thought on the origins of the long cylinder conversions.
Adler sums up by saying, “the prevailing theory is that the 60 known examples were built outside the Colt factory, by an individual or individuals between the late 1860s and
1873-74. Whether or not they were built in Mexico remains one of the unsolved mysteries.”