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There was at least one non-identical ISA slot:

I used to have a couple of IBM EduQuest machines. The majority of the ISA slots were wrong-handed.

IBM was always special. :-) Aren't they the ones who invented the MCA bus abomination that required a floppy disk to configure each card?

You may want to read up on the history of dip switches, jumpers, and plug and play a little bit more.