The tool is essentially the same but is not a "drop in fit".
As the pic above shows, the nail nick is further round the curve on the 84mm version.
This can make it a little hard to get to if you are using it in a 91mm knife.
But I have done it and it's by no means unusable. It is possible to lightly stone the heel of the closing side of the tang to raise the closed position a little to make the nail nick more accessible, but be careful... a little changes the position a lot.
The other difference is the pivot hole. 2.2mm on 84's and 2.5mm on 91's.