We've not gotten any definitive answer, but if Futaba carries through what they've done in the past, then the "airplane" versions will have the spring switch at the left rear, and the "heli" versions will have it at the right rear.
On my desk, I have a, 8FG, a 12FG, a 12Z, a 14MZ, and an 18MZ. Two are "heli" and three are "airplane". They are configured as I stated above.
If you order from your dealer and make your wish known, we can likely swap the switch before the transmitter's shipped from here. Have your dealer talk to their GP rep.