I'm not sure of any racing rules, but here are some pictures showing various sails with numbers. Hope it helps. I couldn't find a picture of showing the numbers on a 222 sail, but as you can see, it is pretty much the same position on any boat, at least proportionally. The pics of the 22 and 23 that both appear to be hull# 1978, were taken from the 1978 Sales Brochure, so I think O'DAY used that number only because of the Brochure year (same pics were used for several years, hey why spend more $$ updating pics if the general look of the boats hasn't changed).