you're right about the gantt chart looking thing for the sequence/timing diagram. Some of the late gtb EM schems don't have it.

the ABC stuff is just the types of switches: A=normally open, B=normally closed, C=make-break

if the letter has the circle above it, the switch uses large contacts.

does your bonus unit step down very rapidly ("machine gun firing")? If not, is the K relay powering and staying powered the entire time the bonus is counting down ... no release/repowering?

if K is not staying powered, can you post a pic of the zero switches on the bonus unit when reset, step 1 and step 2? iir, there's a make-break switch that must be adjusted correctly and the typical problem is when the reset solenoid plunger slams into the coil stop, the vibration opens the switch and the K relay loses power. You get erratic bonus countdown instead of a steady rapid cadence.

if K is releasing you shouldn't have the score reels doing anything when the bonus unit isn't decrementing if all the switches on K are adjusted ok.

because stuff is happening rapidly, it can be helpful just before triggering the bonus countdown to disconnect the score motor ... most games of that era had a "service plug" that let you disconnect. Then you can rotate the cams by hand and watch what is happening.

don't do the manual rotation when one of the 120V solenoids is going to fire ... if you rotate too slowly, the fuse will likely blow on those.

I think the only place I've seen the circle notation is motor switch charts in the parts manuals.

if I had to guess (apparently I do), maybe the * denotes a hold-in switch. I don't recall that being documented anywhere tho.

the /xxx notation on the wire colors is also odd. Maybe it's an engineering / prototype schematic or if that's what was sent with the production games, they didn't bother to redraw it due to the low production amount.

can you post more pics of the schem ... especially the score reel area and whatever relay powers that score reel. The circuit controlling the score reel may be mostly independent of the circuit controlling the bonus unit, so the problem could be elsewhere than discussed already.

yes, the K relay should power and stay powered for the duration of the bonus countdown.

it sounds like you're losing step-down pulses for the bonus unit ... e.g. only one pulse path is getting thru ... or some of the multiplying is done elsewhere and that's not working right.

another test is jumper closed the zero switch on the bonus unit that has one blade connected to the step-down coil. If your bonus unit steps down a lot faster, then that switch needs adjusting to get really good overtravel. You don't want it popping open when the plunger hits the coil stop.

can you take a video of the bonus unit during bonus countdown?