The hull numbers were assigned by the builder Jensen Marine in Costa Mesa California in order of construction starting a 1 and going up to about 1953, the last Cal 20 produced in 1973. I am unaware of the origin of the number you cite. Hull numbers can be found on the inside surface of the transom on the stb. side IF they are there. Not all boats, particularly the early boats have them. Open your stb. seat hatch and shine a flashlite, you may be able to find it if it has not been painted over. California state ownership documents list the number on the title.
Check out the guide found on the C20 web for jumper info. I can send a pix as soon as I take a good one. If you have missing parts Steve Seal is a great resource See links, Call him on the phone if you have ANY questions, he is very helpful He built these fine boats in Costa Mesa and can answer your F/G hull repair questions
Restoring a Cal 20 is a big job but the reward is worth it!