I'm just chucking in some guesses and observations.
The mechanical fit seems to be the simplest answer, as a thread yesterday reported that his card would only insert when "back to front." therefore NOT the card to fit!
The newer cards are rated at about half the voltage of the older ones, and presumably if an old card can fit in a new slot then it will not be damaged, though may give a poor result.........
Conversely trying to run the latest cards at double the voltage, (if it can even be fitted) will lead to tears.