I don't have any PCIe 4X cards but I was able to fit a PCIe 1X card into a PCI32 slot. First time I've tried that... kinda scary that they still mate. (Note the mainboard was not plugged in.
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TommyM: PCI32 (the older style) and PCI Express are two completely different expansion slots which have different connectors and pinouts. If the backplate was on the card I doubt it would have matched. I'm amazed you didn't blow something up.