Two things- if someone else confirms believe me... just sayin, I know a bit about this but not as much as many others here.

I assume that schematic is from the original game, correct? If it is and your running of the Altek board it's not going to help diagnose this- I assume they did the same thing more or less but maybe not quite.

Second- I am not aware of any 43 volt LED at the pinball suppliers. The reason that the LED is only thing lighting is it's only able to pass a little bit of current and the incandescent never heats enough to light. Also- your gonna smoke your LED in a few seconds or less if I am right and your throwing a 6 or 12 v LED in the circuit.

You cannot mix LEDs and incandescents in this series light circuit. You must install four identical LEDs so they each have an equal voltage drop.
The schematic above says they are 921 lamps which are rated at 12.8 volts each.
So you'll need to use four 12 volt LEDs for this application (i.e. car LEDs).

Actually this could get a little tricky because using standard low power LEDs might not draw enough current for the SCR (marked as S2800A on the above schematic) to hold its "on" state. i.e. the four LEDs might flicker.
Your chances might be better if you use multi SMD LEDs which draw more current, though some of them can get quite warm which kind of defeats the purpose of using LEDs.

Anyhow, wish you luck with it.

Yeah looks like you'll need a dummy load.
You could try four 220 ohm, 1 watt resistors (solder one resistor across each lamp socket).

reverse polarity?

OLDPINGUY has a point. Most 12 volt LEDs are directional since they're intended for cars. You need to make sure all four LEDs are installed in the correct polarity.

Changes are you are using bayonet bulbs?

Grab some alligator clip wires and attach to 2 leads at socket and without crossing touch bulb bottom tip,
and the other to the side. Also try reversing this position.

Try this when you expect them to light.

Sometimes even in wiring, they may flip flop and reverse in series.

If there is no code to tell them to flash, you can find 12v blinking bulbs, so that aspect is controlled by the bulb.

There are also blinking boards, that can go in line and do a series after that point.

I cant say if this will always work, many games.....

Socketed extensions from Matrix can help test or install. Binkers, and more, although Ive found the power supply and draw sometimes diminished my ambitious task.

I'd say those #921 12V lamps are used because they're brighter so you get more effect when the game flashes them.

But they rarely flash during gameplay or for any extended period right? I'm not sure there's much to be gained by changing them to LEDs.

I don't know how to explain this without getting technical so here goes.

Those four 12V lamps connected in series are switched via the "S2800A" SCR (shown in the schematic above).

When the SCR is switched ON at the "gate" pin, it will stay ON so long as there is sufficient current flowing through it. This is known as a SCR's "holding current".

The datasheet for the S2800A states the "holding current" spec can be up to 20 milliamps, meaning the SCR will stay ON so long as 20ma (at minimum) is flowing through it. When the current drops below 20ma, the SCR will switch OFF.
Incandescent lamps cause more than enough current to flow through the SCR for it to operate as desired but LEDs generally on their own don't because they consume too little current.

Each of those LEDs in series will have for arguments sake about 12 volts across them.
Using ohms law, (R=V/I) this tells us the resistance required across 12 volts that will carry 20ma in this series circuit would be 600 ohms.

However, 20ma current through the SCR is borderline and we need more to ensure it stays ON when it should. Also because the supply voltage to these lamps is unregulated DC (which is rising and falling in a semi sinewave), the voltage across the LEDs therefore rises and falls and if we were to use 600 ohm resistor loads, the SCR will not have enough current flowing to switch ON soon enough in the rising voltage cycle or will switch off too soon in the falling voltage cycle - these factors help to cause LEDs to flicker.
In this case I usually calculate the load resistance across the LED based on a much lower voltage (about 1/3) across the LED.
So 4 volts divided by 20ma gives us about 200 ohms. When the LED has 12 volts across it, the resistor will have 60ma through it plus the LED will have about 20ma (80ma in total through the SCR well exceeds the minimum holding current spec).
Calculating the resistor wattage is based on the current and voltage across the resistor. In this case the resistor will consume about 0.65 watts so using 1 watt resistors gives you the head room needed so it doesn't cook.

I hope this makes some sense.