That isn't going to be easy, no doubt she is older than 1972... but the exact year will be only an estimate. however, this may help.. I had a 1969 Widgeon (year is a pretty good estimate based on a local O'DAY Dealer's memory.) My boat was Hull# 22698 and Class# 1791. Hull# refers to total number of boats built by O'DAY and Class# refers to how many of that Class (Widgeon in our case). So, I had the 22698th boat built by (or for) O'DAY and she was the 1791st Widgeon built. Yours was the 20088th O'DAY built and the 1402nd Widgeon. I would say that you would be pretty safe listing your boat as either a 1968 or 1969 for build year. (Sales Brochure for 1969 claims "over 1400" Widgeons had been built, so I guess yours is more likely a 1968)
If you would like any Widgeon advice, just ask! I have also created an expanded version of the Owners Manual for the Widgeon, could make you a copy (unfortunately, I do need to ask for $20 to cover printing and postage, it is a thick booklet).
My advice is Free though!
See below for 1968 and 1969 Brochure pages.