I can think of no reason to buy the B-66.

The B-67 functions exactly the same as the B-66, weighs 2 oz. less and costs less (at least wholesale).

The 2 rails of the B-66 are a pain. The double wire saddle clamps aren't made of the good, hard steel of yesteryear, so the serrations don't hold up. And they are expensive to replace. I have the Seat Sandwich and a modern seatpost on one bike and it's an ugly lump of metal that looks out of place

We have two B-66s, two B-72s and five B-67s. Wish they were all 67s

BTW There is now a single rail unsprung version with the same top as the 66/67, the B-68.