Answer:
You won't find any regulation on this question because those old Absence of Endorsement Guaranteed stamps have been outmoded. They simply are not needed any longer.
Take a look at Texas UCC section 4-205. You'll see that when your bank is given an unendorsed check by a depositor, and you send it forward for collection, you obtain holder (or holder-in-due-course) status and make a warranty to those in the collection stream that your customer obtained money or credit for the check. That, I submit, is all that your outmoded stamp ever did for you.