This is how I set up free play on my 1976 Williams Toledo:
I tied a length of thin strong wire (dental floss) to the appropriate
part of the credit wheel, then through a small existing hole at the top of
the machine. When I need free games, I pull the wire and it gives 1/4 turn
up to 15 games. Then I have the satisfaction of seeing the number go down,
then up again when I win! It is more electro-mechanical to wind the
wheel like that than to flick a switch!