Well,,,,13 560's later,,,,not an "E" on the back tang of any of them, other than the Black one that Chuck Buck signed on the front of the blade and Elgin Thomas signed on the back of the blade. But I don't think that is the E in question.
Clueless here.... now off through the ebay sheets to figure out how the dickens I ended up with 13 of those things...

and I never paid any attention, but there are some of those 560 sheaths that are stamped 560, that are the both vertical and horizontal carry...
One of those sheaths was even stolen from Pat Pending. Her name on a sheath of all things.

and no,,,I don't have the missionary codes.