No idea if this is an original part or not, but there was also a thin metal section on mine which goes inside the plastic button housing. Like this.

2014-09-09 07.39.48.jpg

Not a great deal from what i've read elsewhere on here....

https://pinside.com/pinball/forum/topic/rb-v32-roms-anyone-use-in-their-machine#post-512337

Yeah, I think my backbox is all 555's apart from a few behind where the coils are for the hat trick assembly.

I would presume that someone has had faulty bulb holders at some point and rather than get the right ones to replace them they've just used what they had lying around. Shame

Yes it is a diode and there are two of them that I can recall. One is near the Boris Button and the other is on the tilt bob assembly. The photos below are from my machine.

I'll try to have a look but i'm due to go on vacation tomorrow so time is a bit tight.

2014-07-21 20.42.42.jpg
2014-07-22 23.02.38.jpg
2014-07-21 20.22.27.jpg