...other than manufacturers do ship with only their assigned prefixes.

Not even that. I was once troubleshooting a small network for a friend. Nothing worked. It turned out that all of 3 or 4 ultracheap no-name Ethernet cards he bought had the same MAC address. Perhaps it was possible to reassign them, but there was not a sheet of documentation, drivers, nothing (they emulated some popular card, I don't remember which). We just tossed them. Serves him right.