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Back in the day for a very short time, Apple offered a GT120 with a different part number for EFI32 Mac Pros, it's been around 15 years, but I remember that the EFI32 one had a different ROM and some resistors next to the PCIe golden fingers acting as PCIe straps were different from the EFI64 one.

You will have to find a dump of that version. PC version of GT 120 is very different hardware wise from the Apple version and you can't just flash the PC version to the Apple EFI64 one.
 
TechPowerUp have a dump of the normal early-2009 Mac Pro version. Your second link seems more an add for MacVidCards than anything.

I'd get AppleOEM HD 5770/5870 instead of trying to find a card that was sold briefly and with very low numbers 15 years ago, probably only to the Enterprise or Edu sales channels - it wasn't a card like the AppleOEM 8800GT that was sold for MacPro1,1/2,1 as a normal upgrade GPU with a public part number and offered to anyone back then.
 
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