The membership application form
http://daysailer.org/pdf/dsaapp.pdf has the contact info for the DSA secretary. In my case, the state registration # was believable for the range of sail numbers based on the age of the boat (as determined by minor design features, not state records) and so I "claimed" it as a sail # after contacting the DSA.
BTW, it is a known fact that sometimes sail numbers were not given out in any relation to the hull numbers. Two effects combine. One, the hull numbers apparently covered the full range of hulls created at a given facility, not just a single type of boats. Second, in the early days, sail numbers were apparently based on which set of sails shipped with the boat (picture a process where sailbags sit on a big shelf, and, as final step in production, someone grabs a bag, reads the number, punches it on the hull placard).
This changed with the introduction of the HIN (which your boat just missed by a few years).