Sounds like you are right that it is already modded. I took the bolts out of the SM-420 and used them in the T-5. The only difference was the T-5 doesn't have threaded holes on the bottom, so I will need to use a nut and bolt.

1954 Chevy custom 261CID, T-5 5 Speed.
1952 Chevy 6400 2 ton hydraulic flat bed.