91A-5255 would be the tail pipe for 40. the 48- is a bit earlier. but it seems to be working in this application. Most of the time just the prefix for the part number is stamped into the "Service replacement" Ford parts. This is the way the tail pipes and mufflers were identified on the dealers shelf. Very hard to say if this is aftermarket or Ford at this point.
Terry is correct in that there were no part numbers on the tail pipes installed at the factory level. However, factory installed tail pipes have a small "notch' at 3 o'clock on the very end. This was to allow better orientation upon installation at the assembly line.While fooling aroud with my newly aquired 40 the other day I noticed this on the tailpipe.