There isn't one, however some later cars used a clutch interlock switch. There is no need for one and you do shorten the life of the thrust bearing face on the crank if you fire up with the pedal depressed as all of the weight of the pressure plate is on the thrust surface with no oil pressure.
Some cars also used a seat belt safety switch in that circuit.
I actually have a nuetral safety switch on a later 4 speed clutch pedal set-up...........If you have one, it would be on the clutch pedal arm under the dash. To by-pass it the wire coming off of it goes to the voltage regulator, this is were a wiring diagram comes in handy, this wire must be grounded. This is what I did on mine. 0/