I asked someone a similar question recently, and their advice was to check the book " "The J Class Yachts - Enterprise to Endeavour, by Ian Dear".
I also found this on the website jclassyachts.com
"The Trophy became known as the "America's Cup" and was taken back to the USA. Yachts were able to challenge to win back the cup and a series of larger and larger yachts were designed to compete. After a considerable development period the America's Cup Committee announced in 1928 that the J Class was selected for future events. This Class was designed using the new Universal Rule, offering probably the first One Design Class in the World! This defined the size with a new formula:
Waterline: 76' - 87'
Overall length: 120' plus
Displacement: around 160 Tons " ybw.com
has a section on classes, and includes the J-class in here. Their definition seems to confirm the one above