Jason,
#916 is certainly a pretty low number, but I cannot see how it could be before 1971. The first year of sidewinders was 1969. I have a 1973 sidewinder and the sail # is 1181. I have a 1974 sidewinder as well but I don't know the sail number off hand. I know of two other sidewinders, one from 1970 and the sail number is 448 and one from 1977 and the sail number is 1827.
So, your first boat (#916) is probably from 1972. Your second one (#1271) is probably from 1973 or 1974. I wish I could find the sail of my 1974 to help narrow it down a bit more but I don't have it handy.
Paul
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