I'm kind of hesitant to mention this, but here goes. If you don't have a shaker motor installed, Q6 is unused and is of the same type as Q15. Disclaimer: I only mention doing this if you have the proper equipment that will allow you remove Q6 without damaging your I/O board. If all you have is solder wick, a hand held solder sucker, or a cheap soldering iron, forget it. Don't mess up your board removing a transistor to swap into another location. But if you want to get your game going an you have the right tools, this is a way to do it.
I know you've already ordered the transistor but I'll mention this anyway. Ed at Great Plains Electronics stocks the MOSFET (transistor) that you need: STP40NF10L. This is a more robust part than what is currently on the I/O board (40 amp vs. 20 amp). Especially for the flippers it is better to use a more robust part. Stern even started using higher current FETs for their later I/O boards (46 amp I believe).