The original HULL ID Number was molded into the transom, not sure where on the outside of the transom it was located on the 23, Technically it should be located near the starboard side, about 2" below the hull-deck joint (Gunwale/Rub Rail) and be marked in 1/4" characters. On my 1979 DS II the HIN is dead-center on the outside of transom. First pic shows where the HIN was on a 1978 O23 (can't see it, I know, but it shows location)
Second pic is an HIN from a 22 showing format, hard to read, but is is: XDYJ2380M77A, ( looks like a "tag" but the HIN was marked by sticking an embossed label onto inside of mold to leave an impression in the gel-coat)
O'DAY used same format on all models generally. (XDY = O'DAY, J = 22, 2380 is Class#, M77 = 1977 Model Year, A = August 1976, the month the boat was built)