Not a 20, she must be a 21
As a former owner of a 1970 CAL 21, I'd say that the boat is not a 20 but is a CAL 21. The fact that the CAL 20 had a fixed keel, and the CAL 21 was the only CAL model that ever had a retractable keel...plus the HIN that you quote seems to indicate that the boat is a 21 CAB217050374, would seem to translate to CAB = Cal Boats, 21 = CAL 21, 705 = Class# (number of 21s built) 0374 would indicate the boat was built in March of 1974. The 21 was introduced in 1969 as a 1970 model, the 20 was introduced in about 1962. The 21 was built until 1974, the 20 until 1975 or 76. A Cal 20 built in 1974 would have had a much higher class# than 705, but that would be about right for the 21.
The CAL 20 has a raised deck, like the 25, 2-24, and 28, the 21 has a trunk-cabing like most CALs. I'll post a picture of the CAL 21, but if you look in the Model info section of this website and click on 21, you will find much more info that I've posted.