I just found a Jan '65 ABS (hard plastic) shell on a black 500. I've never found an older one. Anyone have input here? Perhaps Paul F. can give me the answer. I did find a black Tenite (soft plastic) '64 a while back, but didn't note the month. I know Tenite continued on black 500s after the colors went to ABS in '59, but I've not seen any black Tenite after '64, or any black ABS before '65.
I can say that the earliest one reported was 1959.
(Although I haven't seen it personally, ;) it was reported by a usually reliable source.)
Thanks Paul for the input. That does seem like a very long phase-in period for new production parts, even for that era. Were there multiple plants making these that might explain it?