These frames will only fit the new type RCD blower starters and not fit the older RCD, big C&A or Davis starters. A new added bonus on the frame it has been machined with a foot to hold the starter from rolling around when it is not in use and sitting on a flat surface.
Center hole is 4.125" and frame mounts to the starter with six 5/16" flat head bolts. Shouldered heat treated bolts lock the starter frame into the starter mount on the supercharger.
If unsure if this frame will fit your application check out #2050-0001 to make sure your starter looks like the one in the pictures.