In addition to the above, Stern fitted a 27k ohm resistor at R21 on their MPU-200 boards.
Quoted from barakandl:
I don't think the resistor change happened until really late in the -35 era and well past the first 7 digit game.
Skateball was the first Bally game with 7 digit displays. The first two pictures below are from two Skateball MPU boards I have with R21 changed at the factory to 24k ohms (presumably using existing MPU boards in stock).
The third picture is from my Medusa, the fourth picture is from a Flash Gordon (sorry for the low quality picture), fifth picture of a board from an unknown source, sixth and seventh board pictures are from Centaurs.
There's enough detail to see R21 is Red-Yellow-Orange-Gold (24k ohms). 36k resistors have blue bands and none of the below pictures have blue in the R21 resistor color codes.
Note the dates bottom right on the MPU boards.
I currently have a MPU board in my Medusa with a 36k resistor at R21 and aspects of game timing are definitely not right.
Skateball MPU board 1
Skateball MPU board 2
Medusa MPU board
Flash Gordon MPU board
Unknown game MPU board
Centaur MPU board 1
Centaur MPU board 2