As others have mentioned, the older "squarish" ones are steel flange, and the star shaped are alloy,
and have removable cups. The alloy ones came a bit later, but are still proper old school, and seem to be the more desirable (read: cost more) than the earlier ones. I have also seen some matching sets with steel flange coaster rear, and plastic hub front, both were Tuff II's and had similar serial numbers.
The cups come in very handy during cleaning and servicing of the hubs..
these cups can be seen on the right side of this pic: