To me it looks they are both the same. The description is the same for both other than the part #'s. The B7 being for the 57 and the B6 for the 56 but other than that,I think that they would interchange. I do see that the suffix letter on the 57 is a C and the 56 is a A.
Maybe that would make a difference. I personally would try the 55/56 straps. According to the measurements above, they would fit ok. Than again, like others have mentioned here, you can clean up the old ones and re-use them. KultuLZ always comes up with great literature on most fords, he is of great help to all of us here in the group.
Thanks Kultulz for always helping out.