UPDATE:
I bought the guitar. But not before I checked with Gibson, Key Music Brussels (an official Gibson-dealer/store where the seller supposedly bought the guitar earlier this year) and an independent guitar technician (who's specialised in Gibson). All three confirmed me it is legit.
However, the volume & tone-knobs were changed and the 'pokerchip' was added to the toggle switch. I got the original knobs from the seller, and everything that accompanied the guitar (deluxe gigbag, booklets, certificate of authenticity, picture,...). I might switch the knobs since I'm not really