US SET is a typical stamping required on military padlocks that are part of a set of KA padlocks. The sets may be for as few as 2 locks and I have seen requirements for as many as 100 in a set. By law most locks supplied must be US made. The MFG number would identify the maker of the locks for the military. To my knowledge those numbers are no longer required on the locks.
I would guess that lock was made between 1950 and 1970 and was likely made by the Chicago Lock Co. or the Fort Lock Co. as they were known for tubular locks and could have met the production requirements of the military. I have some like it that I have taken to local collector shows before and no one is interested in purchasing them.
You may find some new collector whop would want one but I wouldn’t expect them to pay more than $5 for one.
BBE.
