Late to the game. Just a snippet more. I was chasing up Pawnees a couple of years ago. From various registers I got these serial numbers:
1958-1962 0
mid 1962 711
mid + 1962 2000
start 1963 2234
start 1964 2601
start 1965 3131
start 1966 3766
start 1967 4176
start 1968 4498
start 1969 4866
start 1970 5197
start 1971 5306
start 1972 5427
start 1973 7305536
start 1974 7405576
end 1974 7405806
start 1975 7556004
end 1975 7556225
end 1976 7656104
end 1977 7756062
end 1978 7856061
end 1979 7956034
end 1980 8056051
end 1981 8156009
So it would seem Chimbu is close, Piper introduced the year prefix in late 1972 (early 1973 it was definitely in), at least for Pawnees.