jennb,
Yes there are differences. I have a treadle base with a broken center part. My first attempt to replace it didn't work because the replacement center part was narrower than the original.
On my #2 Singer 66 treadle the dress guard I got was just enough smaller to require a spacer so the belt shifter would clear the big wheel.
All of these parts looked the same and until I attempted to assemble them I could not see the differences.
I know of no way to tell if a treadle base part will fit without having in hand.
Joe