I agree with other posters, this is not a J24. Take a look on the transom and see if there is a 12 character code molded in. Usually in the upper starboard but not always on older boats. This code is the HIN/MIC (Hull identification number/Manufacturers identification code). the first three characters identify the builder.
Go to this page on the USCG website
and plug in those first three characters to find out. Every boat built in the US was required to have one of these codes after 1971 (maybe it was 74'. can't remember)