Cool. Curved shape or straight ?I seem to recall an old image of a briefcase with a custom strap that was curved and it looked as if it would be pretty comfortable when carrying a heavy road warrior load in the 257/8.
It could of course have been caused by a small cut in the leather caused by something sharp. This is a high flex area, so over time the small cut could have grown larger and deeper,.It may be worth taking them to a cobbler to see if they can stitch and patch them. Having stitching or a patch on the inside could cause some discomfort if they rub against the foot, however.