Congratulations.

I'm not certain, but I suspect that D4, D5 feed a circuit that is designed to detect the AC waveform of the power input. I've seen other machines where a similar signal is fed to the lamp control circuits to allow dimming effects that are synchronised with the incoming AC. It might be that your LED is passing just enough current to allow this circuit to work, or maybe the machine doesn't need the signal to function.

If it works, it works! I hope you enjoy your pachinko.