O'DAY 25 never had the cast-iron keel, that must have been a integral fiberglass keel. I think I saw that boat on eBay. The O'DAY 25 was built between 1975 and 1983, keel was molded as part of hull, ballast was lead ingots set inside the keel molding, CB was lightly ballasted (15# +/- of lead flashing). The fin-keel 25 must have used a different hull mold, and had a bolted-on lead fin keel.
Oh, and lets make one correction, O'Day NEVER built a "swing-keel" boat, they built centerboard boats (ex: Widgeon, Javelin, Day Sailer, Mariner), they built Keel/Centerboard boats (ex: 192, 20, 23, 25), and they built Fixed keel boats (ex: 27, 28, 272, 240). What retracts into the keel on a K/CB boat is a centerboard, a "swing-keel" is a HEAVY, ballasted fin that can be retracted like a centerboard and usually contains all of the ballast. A Catalina 22 is a swing-keel, the Macgregor/Venture 17, 21, 22, 25 would be a swing-keel. O'Day went big into preaching the advantages of their K/CB boats over the popular swing-keel designs of their competition (they also admitted the disadvantages....but barely!)