Dumb, geeky, possible theory... shell was wrapped black in '66, but since the color was not popular it was badged and sold later?Very good. Oddly enough I have recorded an example of an Apr. '66 date stamped drum with a similarly high serial. Quite an outlier as 80xxx's - 90xxx's is typical for early '66. 145xxx would be expected as '67. This one is particularly interesting because it is a higher than expected number on an earlier drum. The OP's set is lower than expected numbers on later drums.
Not sure if they "pre wrapped" shells