Ed, while I don't have the answer to the "when" question, I can provide some data. I have 7 sets of original (based on single crimp and hose characteristics) hoses. The one from my Dec 66 build included. Mine and 5 other pairs have no part number stamp. Each hose does have CYL on one end, and were attached to the piston CYLinder end. The remaining set has the part number on only one of the two hoses, and carries a date of 0718.
Your questions and my observations lead me to believe that "early" had no date code, "late" did, and we should try to figure out an approximate date for the change. Again my Dec 20, 1966 did not have the date, the hoses appear to be original, but I can't prove that. However, if they were originally date coded one could expect that service replacements would be as well