Yes, it's by gear thickness.
-00: .187"
-0: .345"
-.5: Don't remember. It's .400-something.
-1: .600"
If there's not a number stamped on the outside of the pump next to the CAST-IN "80A", then it's a -1.
Anything else will have a 0, 00, or .5 stamped into it. These days, even the "1" is being stamped in to avoid confusion.
Spud