I think you're underestimating the demand but it's more likely that somebody figured out that the ones they've been making for a B body were also used in a C body....
(And no I don't know that for sure)
Those repops have been around for a long time.
This works in 65-66 as well if you are not a stickler for originality and eliminate the separate washer pump in favour of the in-tank unit.