Unlike everyone else, I hadn't forgotton this thread!
I was looking at the big long number code Fujek posted, if it IS some kind of telephone / polybius square thing and that chap who said that GOG codes don't contain 1s, 0s, Is or Os, was correct then I was initially weirded out that there was a single '1' as one of the groups so I tried a 'Rot5' on the numbers, which looked quite good because 5 would ROT to 0 and there are no 5s and I suspect there's no letters on '0' (although it doesn't explain why there's no 0s in the original message though)
But anyway, messing around with the ROT5ed numbers in a sequence that put '1ABC' on 1 '2DEF' on 2 etc didn't help, this scheme only puts '9YZ' on 9 though
any comments Mr Fujek?
But anyway, those are my thoughts, whether right or wrong.
Something you mentioned is actually a good trail, while the rest isn't too close ;)
Btw. both games (TEX MURPHY 1+2/DUKE NUKEM 3D: ATOMIC EDITION) aren't redeemed even after all this time ;p
Edit: Sneaky posting while I was still writing up mine :p
(I'm guessing the gog-codes all follow a similar pattern to the last one I got: letter, letter, letter, number break number, letter ...-> LLLN-NLLL-LNLN-LLNL)
I can assure you that this is not the case from just the few gift codes I got myself.
The point about including numbers is smart though ;)