Sounds like a 1979
If the HIN reads XDYPxxxxM79m, where XDY = O'Day, P is the model code for O'Day 20, xxxx = the Class# (sometimes called hull#), M79 = 1979 model year, m = there will be a code letter here (A = August, B = Sept., C = October, model year ran Aug - Jul., so A = Aug 1978, L = Jul 1979)However, the O'day 20 was not built in 1979 model year.....so maybe this is not an O'Day 20?