I have gotten a 4 Million BC (German) and am getting the bugs worked out. Getting pretty close. It's in great shape. Here's my problems.

The easy problem:
I have one problem when playing a three player game.
A one player game, two player game and four player game plays fine. But on a three player game it isn't advancing from Player 3 ball 1 to Player 1 ball 2. Instead the sequence is like this:
Player 1 ball 1
Player 2 ball 1
Player 3 ball 1
Player 3 ball 2
Player 1 ball 3
Player 2 ball 3
Player 3 ball 3

The hard problem:
This machine is missing the Advance Ball Assembly. This is the assembly that locks the ball and stages it down for multiball. I found pictures of the mech and it looks pretty complicated. Unobtainium, I imagine.
Does anyone have a parts machine perhaps that has this unit? Fingers crossed!
Thanks!

Yes, it's the tar pit mech. The entire assembly is missing.
There's three little stand up posts that when the coil energizes, it drops the ball down to the next stand up post, until it goes all the way through back into play.
Advance ball assembly (resized).jpg

I rebuilt the player unit today, along with everything in the backbox. There was a bit of dirt there, but it's all cleaned up and looking good (in the backbox). Major clean-up, but the issue persists.
Next I'll pull out the base board and start tackling all the switches and relays there.

Wow! That's exactly what I need! PM sent your way.
Thanks!

I've cleaned up the base board. All relays and switches. Verified contacts are good.
I verified that when player 3 ball 1 drains, the Player Reset Relay does engage as it should.
This makes the ball count advance solenoid fire and the ball count advances. But the Player Unit Up Reset solenoid does not fire.
This is just on a three player game for ball 1. Any other combination works fine. So, that makes me think the relay switches are registering okay.

Same situation occurs on a five ball game as well.

Does anyone have any suggestions?

I took the playfield out so I could get a good video of the relays. Plays great in slow motion on my iphone, but not so when on the computer.
Anyway, here's what happens on a 2 Player game, Player 2, Ball 1:
Ball index - on prior to ball drain. Then when the drain happens:
Outhole relay on
3rd ball relay on
Outhole relay off AND Ball release relay on
ball count unit steps up AND Ball index relay off AND Player reset relay on
Ball release relay off AND 3rd ball relay off
Player Reset off

In this scenario it works correctly. Oddly enough it has very rarely worked correctly like this on a three player game. But 99.9% it dosen't.

Here's what happens on a 3 Player game, Player 3, Ball 1:
Ball index - on prior to ball drain. Then when the drain happens:
Outhole relay on
3rd ball relay on
Outhole relay off AND Ball release relay on
Ball count unit steps up AND Ball index relay off AND Player reset relay on *** AND Ball release relay off AND 3rd ball relay off ***
Player reset relay off AND Ball release relay On and then Off.

So, I gotta figure out why the 3rd ball relay off AND Ball release relay off are happening too soon.

This not normal scenario has happened a couple of times on a four player game with player 4, also.
Testing releays (resized).jpg

Whoop whoop! Got the ball in play problem fixed! Thanks for the walkthrough, MarkG.
Studying my videos, it looked like 3rd ball relay and the Ball release relay were dropping out early. So, I spent a lot of time last night pouring over the schematics. It looked like if SCM 6F wasn't latching consistently, that could possibly be a problem.
So, this morning I checked it out. It was making contact, but not as solidly as I would want it to. When it makes contact it should ever so slightly bend that blade just a little. So, I adjusted it to make a better contact, and now it's working great!!!

Zacaj has the Advance Ball Assembly coming to me soon. It's alive!!! Looking forward to getting this thing restored!!!

SCM-6F problem (resized).jpg

Fixed!
The SCM 6F switch resolved my ball count/player problem.
Big shout out to zacaj! He sent to me the Advance Ball Assembly unit that he had. I received it in the mail yesterday.
Just took a few minutes to install it this morning. Now everything is working 100%!
Whoop whoop!
Now I will be moving on to a deep clean-up, disassembly, etc. I'm going to do a total restore on this machine. It should be a beauty when I get finished.
(Will take a few months, I imagine, but it will give me something fun to do over the fall and winter.)
I'll post some pics when it gets finished.

Thanks again, zacaj.
Cheers.
Duane
Advance ball assembly-4 (resized).jpegIMG_9460 (resized).JPG