XDYT0321M78J
XDY= O'DAY
T = 23
0321 = Hull or Class# This is the 321st O'DAY 23 (I'd say 321st of the so-called 23-2 or 1977-85 version)
M78 = 1978 Model Year
J = May, the month the boat was completed, May 1978 in this case.
I have rigging instructions (the original sheet supplied by O'DAY, also available from D&R Marine) for the 1978 O'DAY 23, but they would only cover the standard deck-stepped, hinged mast. I know that a fixed, Fin-keel was an option (though few were built as far as I've heard) on the 23 at that time, I have never heard of the 177-85 version of the 23 having a kee-stepped mast. The original Pop-Top 23 (1971-early 73) where the whole cabin trunk raised, did have a keel-stepped mast, the non-pop-top model of that time had a deck-stepped mast. I'd suggest contacting Rudy at D&R Marine (508) 644-3001 to see if he has any record of this boat and whether the mast was factory-original. The basics of rigging will be the same as for the stock, deck-stepped mast I would assume, but obviously the actual process to step/unstep the mast will be different.