Help: what is the keycaps material of NEC PC-8801 (Type-A) keyboard?

[GlennL42]

Greeting!

So I just received my NEC PC-8801 Type-A with Blue Alps and is now planning on a restoration job, while doing so, I started to question the material of the keycaps. Before I got the board, I made an assumption that the caps were made with ABS material, judging by the relative heavy yellowing of the caps in the product picture from the website I bought it from, but now that I've received it, I'm starting to suspect the keycaps are made with this colour in the first place, check here for details.

Due to the scarcity of the pics of this specific keyboard, I cannot validate whether caps is yellowed or that they come with this color in the first place myself, so is there anyway to validate this or does anyone here has the answer?

Thx in advanced.

[genesisx]

I think in a previous discussion about retrobright, chyro brought up that sunlight is not the only cause of yellowing, heat can be too. So the inside is yellow doesn't approve anything