Before the requirement for the 12-digit Hull I.D. Number, O'Day gave every boat 2 numbers, a Hull# and a Class#. The Hull# represented how many total boats O'Day had built, and the Class# represented how many of that Class O'Day had built. So your 1967 DS I was the 15344th boat that O'Day built, and the 2724th DaySailer. If there is a number on your sail, it should be the Class# 2724.
This was O'Day's method of serial numbers for their boats until the 12-digit HIN went into effect on November 1, 1972.
Rod Johnson, "SUNBIRD" (rjohnson24-at-juno.com)
According to Rod Johnson, my DS1 is the 2303rd DaySailer built which looks about right for the 1966 date....
