While it's been documented before, I thought that a video showing the 'ghost keys' on a 1988 HP-12C (USA production) would be interesting. You can clearly see the HP-11C key legends as the light angle changes. BTW, the keys are definitely double moulded. Had a look with my son's 100X microscope and I can clearly see the 'shared' plastic border where the two plastic meet, while the blue text is clearly raised, printed over the key. While it appears the key legends are overpainted with this 'ghosting', it's definitely a characteristic of the moulding process.
Link to video is here: HP-12C Ghost Keys Video