for the ones that have more-or-less the answer I was looking for:
1] it can be better than one free ball. Partially shooting a ball up the runway and using the manual lift button can get you multiple balls in the runway...then reset the game and see how many balls you wind up getting onto the playfield without needing to play for extra balls
2] yup - take out a leg bolt and you have a nice hole into the cabinet. Imagine how much practice it took to make use of that hole, then someone comes along and sticks a metal plate behind it.
3] hmm ... maybe a hint. The change I'm thinking about has to do with the way the search relays are mounted.
4] increasing the line voltage wouldn't make coils activate, but lowering it just enough that the replay register step up coil works but the replay counter step-up coil doesn't sounds profitable ... and didn't everyone carry a variac into a bar in those days? Apparently the technique was to play until the game started registering credits, then dial down the voltage until the replay register barely stepped up.
5] from a purely statistical and financial viewpoint, the best way to play a bingo is one coin. You get at least a 4 for 1 return on your investment. At no other time is the return for a standard inline/section 3 win guaranteed to be that good. For the sake of argument, assume that intentionally making any hole on the playfield is equally hard ... in reality some numbers like 16 are harder, but some players are also good.
If you are playing a game that has 300/600 payouts for the blue section, you can beat the 4 for 1 return if you can enable the blue feature in 75/150 coins. The odds of lighting the blue for 300/600 is better than that, and was so much better that bally had to adjust things....
you got 3-in-blue for 300 when enabling the screen to move to at least the F position. 3-in-blue for 600 needed the blue score booster relay to trip and 2-in-blue for 600 needed the super blue relay to power.
ever notice a blue adjust plug and mixer #2 relay switches showing up in the super blue and blue score booster trip relay circuits on circus queen after game serial 1499? Of course not...who looks for that kind of stuff, but it can be interesting to see the differences when multiple schematics are available and try to figure out why the changes are there.
I don't know if the number of spotting disc rivets changed before/after (I think I've only seen one manual revision). It's possible the adjust plug added more spotting rivets to compensate for the addition of mixer #2 relay, or they could have moved some of the original rivets to the plug-connectable path. In any case, the addition of mixer #2 relay switches made it less likely for the blue relays to activate if the screen was already enabled or other features were lit.
on lido and after, they moved the source of enabling the blue relays to a circuit that wasn't improved by the use of the green button, and made it so the blue relays couldn't enable unless the magic screen unit was on step 5,7,9 or 10 - those are unit steps in between D-E, E-F, F-G or when G is enabled. On earlier machines - including bikini - the blue relays could power anytime that any feature could enable - even when the screen wasn't enabled. You could hear them power, but no visual confirmation until the screen was enabled to move to at least F.
so in short, the best return was resetting the game, playing red button to get the green scores to 6 and then playing the green button (if it had one) to get the screen to at least F. If you happened to get the blue/super-blue relays to power along the way, even better.
why do green scores need to be >4 ... 'cause all the screen games disconnected the feature power until the green scores were at least 6.
note you can get features on the first cycle after reset if you use the red button. The scores will step to at least 6 guaranteed, and due to switch timing, features can enable after that step happens. If you reset and immediately use the green button, the scores won't step and mixer #4 relay is guaranteed to power and disconnect the feature advance circuit.
an operator/bally field tech/bally slot designer would modify all his games so blue scored as 5-in-green. He could not profitably operate the games with a 300/600 payout without making other mods or disabling the reflex unit to make the screen advance much harder.