Good Evening, Joan,
The easy part first: Having a separate lens board for each focal length lens is not absolutely necessary, but it certainly makes switching lenses much more convenient. Having different focal length lenses for different size negatives is standard practice. Usually, it's 50mm for 35mm negatives, 75-80mm for 6 x 6 negatives, 90mm for 6 x 7 negatives, and 100-105mm for 6 x 9 negatives. Using a shorter than usual focal length lens normally causes vignetting of the projected image.
The lens board pictured has two additional screw heads(???) beside the lens, not something on any of my Beseler boards, nor on any I have seen. It could well be that someone has, for some reason, adapted the board to deal with some problem I can't think of. I did an recessed adaptation of the board for my 50mm El-Nikkor to move it closer to the negative; otherwise, I couldn't achieve proper focus. That does not appear to be the case with your board. Sorry I can't be more helpful, but I'll bet that someone here will be before long.