sill plates/arm rests

Here is the part number that is on them 1233 51861 3. Is there anywhere that I can look these up to see what they go on?
I think those are early....like 63/4 maybe? Google the part #, also take a measurement screw-screw.
@toolmanmike may know.
After a little closer examination I see where one of the mounting posts have been cut off on each one of them apparently modified to fit.