If you have a steel centerboard on your 23, it must be a replacement, not original. All of the 23s had fiberglass centerboards, however due to the nasty ruting problem of the cast-iron keels on the 1971-75 models, several owners apparently replaced their FG centerboards with steel plates due to the thinner material fitting better in the rusted cb trunk.
The 1977-85 23 was a better design and had as Joe mentiones, a fiberglass keel that was lead-filled and a fiberglass centerboard (I think the cb is the same for all 23s, both 1971-75 and 1977-85, check with Rudy to be sure).