According to this:
The 500RPM motor is for the 230v countries.
The Stainless Steel/Acrylic top plate was on the MKIII and a few of the first MKIV HW-19's. I was a VPI dealer and Harry told me that stainless steel became too expensive and they switched to a black metal/acrylic top plate later on in the MKIV version.
I upgraded a bunch of these tables, way back in the day for people. I know the HW-19 tables very well :-)