There were only 4 squares for 4 ranks. With no half squares or anything like that.
But that would be identical to the TNG scheme, only with squares instead of circles. Use black squares mixed with the silver ones, and you get the exact TNG rank scheme.
Just because TPTB didn't bother to create those black pips means little. They didn't bother to create the Lt.Cmdr pin for ST2, either - but that doesn't mean Starfleet discontinued the rank.
Timo Saloniemi