Neo geo or MK3's run button. I actually use the seventh button for turbo in nba jam games.
stop stealing my answer

also, I dont use the top row for my jamma (3 button) games, I use the ABC of the "neo geo" row, its just better that way.
The reason for no coin buttons:
a)we use coin mechs
b)with an Ipac you can hold start and push button 1 for a credit
The reason for no exit button:
you can hold P1 start and push P2 start(IPAC)
The reason for no enter button:
most front ends use P1 start as enter
The reason for no pause button:
you can hold P1 start and push down to pause (IPAC)
I mentioned IPACs because thats what I have, Im sure other encoders have the same/very similar options. MGB posted a frankenpanel, if you want a better idea of what people here use (both good and bad) check out the control panel picture thread.