you can see the outline of the longer slipfitter of the older light since there's a couple inches of no rust on the mast arm near the M-250R2. The old light doesn't seem to have been a 60s GE since the rust line would go the other way but maybe it was a 70s GE or a OV-15?

I think I've seen this happen with truss poles here which used to have B2213s

This is a Line Materials arm, and in Boston area a lot seems to hold the Line Materials Unistyle 400 so it may have been one of those!

since this light is 100W though maybe it held a unistyle 175?