I would believe that the bracket was on all cars becuase the "body" didn't know what type of car it was going to become when the body was built.
Based on the AIM, it appears that this bracket was only used when a single exhaust car went down the line. (AIM UPC 8 A7 page 120) On dual exhaust cars the bracket was still there but unused, and the LH frame welded bracket was used to support that side of the system. (AIM UPC 8, A8 page 121)
The AIM drawings do not look like the chambered exhaust system used this bracket (AIM UPC 8 A9 page 122)