Can you post an audio recording of the full melody showing the missing last tone?

Unfortunately you can't post them here. Try uploading to youtube or some other external web site, even as a MP3 if you have to.
BTW, the last tone (you say is missing) is the same frequency tone as the third last tone in that melody.

Cheers, you're getting too many wrong tones - you have a logic problem.
Ok, check the easy thing first. Pull out the sound board and check the soldering on the J1 pin header. It sounds like you might have a bad connection on one of the sound select signals coming from the MPU board.

BTW have you reterminated any connectors on this game? Any battery corrosion at the MPU board?

I just tried it in simulation and can confirm your sound board is missing the sound select signal at pin 3 of the J1 connector. So it's either cracked soldering on that J1 pin header, a bad crimp terminal at pin 3 of that J1 connector or if you're unlucky, you have a faulty U1 or U2 chip on the sound board.

My guesstimate is that someone wired the #1 bunny key lamp to the wrong location on the lamp driver board.
It comes from J1 pin 23 of the lamp driver board and is a white-brown wire.
NOTE, the connector pins at the lamp driver board are numbered upside down, so for example pin 1 is at the bottom of the connectors and counts upwards.

So check that the white-brown wire at lamp driver board is in position 23 of the J1 connector and also that the #1 key lamp under the playfield has a white-brown wire soldered to it.

Note, the video above is in attract mode and that #1 lamp isn't lighting so it's not a switch issue. The #1 lamp should come on straight after you see the red specials key light illuminate.
The fact that it works in lamp test mode says it's wired to the wrong spot at the lamp driver board.

Great to hear!
Just for clarity:

LDB_J1a.jpg

Have a look at @inkochnito's backbox pinball cards for Playboy:
http://www.pinballrebel.com/pinball/cards/Backbox_cards.htm

The shorter stiff centre leaf is there to hold the idle lower leaf position (the lower leaf should be tensioned against it) - the centre leaf isn't there for any electrical contact reason.
The purpose of that short centre leaf is replicated on other playfield switches.

It's a known fix for the common hum these games have. Many people (me included) have done it on their Playboy, Kiss, and other machines with that sound board. You've got nothing to lose by trying it.

That ebay backglass you refer to looks like a CPR latest run glass. The giveaways are the rounded corners on the glass and the thin mirroring around the Bally logo.
The mirroring on my factory backglass around the Bally logo looks like this one - like a 3D effect:
https://www.ipdb.org/showpic.pl?id=1823&picno=54723&zoom=1

Use a small flat head screwdriver to adjust the volume control potentiometer.

Playboy_SoundBoardM.jpg

Someone has obviously swapped in a transformer assembly from an earlier Bally.

There's supposed to be a bushing on the screw side:
This is not the right one but it's something like this as an example. See the bushing picture two posts down.

With the screw on tight, the bar rotates freely on the bushing.

Ignore the brass washer seen in mine, it was just to reduce excessive slack.

IMG_0043.jpg

Sorry, excuse my bad memory. I took it off and it's a bushing plus washer (washer goes against the wood).

IMG_0010a.jpg
IMG_0005a.jpg
IMG_0002a.jpg

I have an old Canon IXUS digital camera (10X optical zoom). Those pictures were taken about 3 inches from the object in the dark with the flash - usually I much prefer to take photos outdoors in shaded daylight for maximum natural light on the object to get the best clarity.

Your lock bar looks correct to me. BTW make sure there's no junk sitting in the lower head channel where the glass sits that could be causing the glass to sit higher than it should.

IMG_0042a.jpg
IMG_0031a.jpg
IMG_0014a.jpg
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IMG_0013a.jpg

You will quickly get frustrated by the playmate target switches not responding to fast hits. Install the 0.047uF capacitors, no question.

An operator cut the capacitors because they likely failed (common problem) - for them it's better to have the game playable and too bad about fast hits.

On Playboy, switch matrix capacitors are required on the:
- five playmate standup targets
- three pop bumpers
- centre 5 key standup target
- star rollover in the extra ball/special arc
- tilt plumb-bob

Yes, my untouched Playboy cabinet is white first, then magenta then black.

Got any more internal pictures showing what you did to the cabinet?

Thanks for sharing, clever indeed!

I presume you are/going to build some kind of rack to store them?

Would I be correct in saying the right slingshot fires when the drop target reset coil is suppose to activate rather than the outhole coil since they happen at almost the same time at the start of a ball?

If yes, the other pop bumper error you mentioned points to a loss of solenoid select signal at the solenoid driver board on J4 pin 6 (blue-yellow wire). This wire is daisy chained from the sound board connector but the problem is more likely at the solenoid driver board since you moved the cabling.

This is the beauty of Bally drop targets. You need precise aiming to knock them down. They will make you a better player.

Also check that the ball is going straight up the grotto and not ricocheting from side to side causing loss of momentum.

Have you considered repairing them? A good chance it will cost pennies. Check the soldering on the connectors pin headers first, the solder joints commonly fracture/crack.

With the machine off, disconnect the J2 and J3 connectors from the MPU board and the J4 connector from the lamp driver board. Power up. Does it exhibit the same display problem?

Can you post a picture of the original Bally MPU board - need to see if it suffered battery corrosion and how far the corrosion reached.

Hmm, not much corrosion on the board, pity about the poor rework on the U10 socket replacement. The connectors to the MPU board likely didn't suffer any corrosion damage from the MPU battery. Still doesn't mean they're in perfect condition.

What happens when you swap the displays around? Does the issue follow the actual displays or stay at the display position?

With the machine on, what happens when you wiggle the J1 connector (top left) of the MPU board? Do displays 2 and 3 flicker with the correct info?

Sounds like there's a job ahead of you to re-terminate that J1 connector. Wiggling it told you there was a connectivity problem to be fixed.

All the connectors on the MPU board are 0.100" Molex. Re-use the original housings unless they're damaged, the longer ones are difficult to find.
The crimp terminals are held in the connector housing by a little retaining ramp. Use a small screwdriver to push each ramp down then pull the wire/crimp out of the housing. Here's an example video on YouTube: