So I was going to take advantage of the offer to activate my disk versions of the Stalker games on GoG, and while the first and the third have perfectly legible keys, the second game has an atrocious dot matrix quality key printed on the back of the manual that I had a really hard time deciphering.

So, fwiw, if you have a letter that looks like it might be a "6" or an "S", and neither are working, it's probably a "G".

And I realise this sounds stupid, but it took me fifteen minutes before it occurred to me it was a "G", so this is just a public information announcement.

Oh, also you need to enter the letters as upper-case for them to work.