HELP NEEDED WITH 50's AMI SELECTION PROBLEM

I recently rebuilt an AMI model 'i' 120 electric select. I went through the mech carefully setting up as per the manual and everything works fine apart from one fault: whenever '6' is selected (with any letter), '7' is played - e.g. select B6 you get B7, select E6 you get E7 and so on. All other selections work fine. I could probably eventually find the fault, but I was hoping an AMI expert might be able to give me a quick lead as to where to concentrate my investigation. Many thanks to anyone who responds. Paul in UK.

For anyone who's interested in what it was: I noticed that when any '6' was selected, the stepper received an extra pulse (making '7') which coincided with the button bank reset bar being actuated. When I turned the pulse generator by hand I discovered the outer wiper blade was not quite clearing the last stud on the disc before the buttons reset and this was setting up some strange kind of 'shunt' giving the number stepper the extra pulse. By advancing the wiper blade a tiny amount to clear the last stud the fault was solved. Q.E.D!