Last summer, I decided to replace all 4 rubber boots on my Mark II, BT7, and ordered four 3 1/4" boots from my favorite Moss distributor. The Moss website showed both the 3 1/4" (#280-520) and the 2 3/8" (#159-600) as being replacement boots for the "594" lens. My new boots were longer than the old ones (believed to be the original ones with the car), but each lamp unit, ring, and lens taken from an old, short boot, easily fit into each new, long boot. I replaced the boots, and the only differences that I see is that more of the wiring is inside the boot, and the longer boot protrudes further inside the fender.
I believe that the 2 3/8" boots are correct for the BT7, but the 3 1/4" boots will work. Several years ago, I had a Mark II BJ7 parts car, and I think that it had the longer boots as original.
Jerry