3 Slot Payphone

This is a 3 slot payphone, made by Western Electric. This is chrome with a green dial and a green handset. You would not have dial tone until you deposited your money and if you stayed on the phone longer than you should,the operator would ask you to deposit the extra coins. She would be able to tell the amount you deposited by the different sound the coin would make hitting the bells. Nickel one bell,dime two bells and a quarter different altogether. Does anyone know what keys I need for the upper housing. I believe they were common for the repairmen. I know the coin box key was different for each payphone. I appreciate any info on upper housing key. Thank you