Both the 784-8 and 9 have to have matching software to be compatible. Different versions will not work to my knowledge
You know I don't think that's how it works.
I think the fx784-8 comes populated with solder on eproms that have a base program that runs all the mazatrol versions and the same with the fx784-9 board.
It's not until you install into the sockets the labelled eproms that you get a T2, T2 with EIA, T3, T3 with EIA and so on.
This is my theory.