Finally found a Geforce PCX5900 (PCIe Geforce FX5900). This is the higher clocked Gigabyte model. GV-NX59128DP Turbo Force edition. This one is still in the shrinkwrap though it will not be for long.
From what I could find, the PCX5900 uses the FX5900XT GPU and the supposed clocks match that.
Seller's pic:
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Pic of card from Newegg:
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This gives me almost the full range of the Geforce PCX cards except for the lowest, the PCX4300, which I am not even sure ever existed. That and the PCX5950U which may have also never been sold.
So I'll have the PCX5300, PCX5750, and PCX5900.