NavList:

The A-10A added a a very primitive escapement that sent a pulse to a relay which then sent a pulse to a solenoid that pushed that pencil lead to make a mark on the disk. In use you hold down a trigger and the averager sends a pulse every second to the solenoid so 120 marks in two minutes. Then center the marks agains the pencil lead and read out the altitude..

So that means they had to start at one end and without disturbing the paper disc count up to 61 and mark the 61^st mark a bit bolder (or just move the marks against the resting pencil lead until they’d counted 61). Wow! I’d lose count. Is there a mathematical proof to say that the median value also corresponds with mid-time other than saying that if you have 121 equi-timed marks the median has 60 equal time periods on either side?  I'll put some batteries in mine and see if it works any better.  DaveP