On a Gottlieb 1973, for the two coin chutes I have one Jones plug that adjusts the credits for only one of the two chute.  My first chute is fixed at one credit per coin, and the second shoot is adjustable by the Jones plug.

That Jones plug is located on the bottom board of the game, not the back box.  It is roughly centred on the board (regarding left/right), and 3/4 way towards the back of the machine.

There is one pin (single pin plug) that can be moved into any of about 5 sockets.  Each socket is a setting for the number of credits for that coin chute - simply place the pin into the socket that represents the number of credits you want.

Sounds like you have three of these Jones plugs.  If you still have your cards in the game that identify what the comments are, look for the word "chute adjustment".


(Note that 'chute' is an alternate way of spelling 'shoot'.  My card used the spelling 'chute').