The main obvious difference is the 28-29 boards are Straight (the front edge and the rear edge are parallel and the board is a rectangle.
The 30-31 board on the other hand is not a rectangle and from front to rear it gets wider in the rear. There are other differences but this will immediately distinguish one from the other
Thank you, some friend offer me 1 pair from 1930 model Tudor , for my early 28.. I know there was a difference, but not what the diff was.. But i know now , they will NOT fit on my Tudor.. Thanks,