System 1 games will tend to put gibberish into the memory for high score and extra ball tiers instead of clean zeroes. You can access those with the test switch in the door. To clear them:

Press the test switch in the door for two seconds. Credit should go to zero, and a figure should be on display 1. It will likely be garbage, might be zero, might even be a recognizable number. The Credit display is just letting you know which memory/test you're looking at. Press the button on the MPU, that will reset that memory to zero, then you press the test switch to save it and move to the next memory. Do that until you get to 11 on the Credit display, at which point you'll get the display test for displays 1 and 3. Let it cycle then until it resets. IIRC tests 0-6 is auditing, 7-9 should be the extra ball tiers, and 10 should be the high score to date. 13 is the solenoid and light test, but the light test portion is effectively useless except to verify that "yes, that light is indeed out that I noticed during play".

The battery pack will matter, in terms of avoiding having to do the memory reset every time.

Additional: In order to set the "scores for extra ball" and "high score" manually, when you use the test button to get to those locations, use the Start button to add 1,000. Basically, set it to the score you want, then press the test switch as before to advance to the next location and save.