The one time I bought a couple, we negotiated the price and as I recall it was about 30% below normal wholesale. Broken braces, big wood flaws, knots, neck that would not adjust, etc. My '67 ES-335 has the "2" stamp very lightly impressed under the serial.
I don't know that we can declare a certain dollar value or percentage deduction.I have never seen a Gibson with CULL stamped on it and know nothing about it. Only if all else is equal and you have two otherwise identical guitars side-by-side. After 40 years, whatever made it originally a "2" is no longer relevant. The dealer didn't mention it, and I wonder if he even saw it. This discussion interests me because I sometimes wonder if I paid too much. But I would quiet that nagging voice if I were you."SECOND" maybe yes because the stamp is so god awful ugly. The guitar simply wails, so I'm very happy with it, but again, the "I should have paid less" voice sometimes nags me. The "2" is really irrelevant at this point because remember it only meant a 10% difference in the wholesale price to the dealer 42 years ago. Given what the market is for vintage instruments now, that and the "2" just has no relevance to current values.Gibson did have another category of imperfect instruments called BGN guitars.In general these guitars were usually seriously fucked up and should be avoided unless you are clearly aware that whatever the problem was was fixed.To answer your theoretical question about two identical guitars except one stamped "second" or "2", then I would say YES the stamped one would be worth less.