The plastics used truss head screws in the wooden side rails. Pan heads were used on the posts.
This is the general rule though. It seems Bally assembly deviated from time to time (probably when they ran out of a certain size/style or received a different lot).
Bally used #6 truss heads on some machines and #8 truss heads on others. You can use #8 because they are only 0.030" bigger in diameter and the holes that the original #6 screws were in are usually loose. My bolts have the same head size but will fit nice and tight like new, especially for the backbox wire looms and displays which used #8 a lot of the time. I do offer #6 truss heads too if people want them.
You have to remember that you can polish the OEM screws and they will look good but the appearance is usually short-lived because they were just cheap zinc-plated fasteners and the zinc wasn't very thick. They will usually oxidize quickly afterwards if left unprotected.
I've studied all the Bally fasteners in depth and manufacture several custom screws for pinball. All of my fasteners are 18-8 stainless and can be polished to a mirror finish with a slight buff or left natural to appear like the original zinc-plated bolts were. The lengths are custom and the heads are very close to the originals. Mine actually look better than original and are superior quality. Unlike the cheap originals, mine are designed to last forever (and look good too).
Custom sized 18-8 stainless steel fasteners from left to right:
1) 6-32 x 2" pan head (for posts with threaded T-nuts beneath the playfield)
2) #6 x 1.75" pan head sheet metal screw (for tall playfield posts that screw directly into wood)
3) #6 x 1.5" pan head sheet metal screw (for short playfield posts that screw directly into wood)
4) #8 x .5" truss head screw (for playfield plastics & backbox wire looms)
5) 8-32 x 1.125" carriage bolt (for Bally coin door frames & side rails)
6) 1/4-20 x 1.5" carriage bolt (for Bally playfield glass lockdown bar lever)
I have all these screws in stock and am adding more this year. You'll have a hard time finding these screws because they are not popular sizes. Also I made a point of trying to match the heads as close as reasonably possible. Carriage bolts you buy commercially will all have marked heads. Mine are smooth and I spend thousands of dollars having all of these screws custom-made.