I decided to skip the deal, there was some funny looking discoloring on the upper half on the back of the headstock among other places. A bit suspicious, like if the serial number might have been fiddled with or something, especially on such an expensive guitar.
The SG 61' RI is very neck divy, but it does have Grovers. Never understood the deal about putting Grovers on everything, my LP Special DC from the same year also had Grovers when I got it... The strap button was placed at the SG-position, which was pretty much useless. Moved it and everything felt much better. Replaced the tuners as well, and then it felt just right! Overall weight was also noticeable lower! The issue with the SG strap button placement however, is not only the balancing issue, but it feels very weird and unnatural to me, even if it would have been perfectly balanced. And even though it rarely happens, it is not very nice to have a strap button poking me in my belly which is full of scars after three significant surgeries. So it will get moved next week, and we'll see how it works out
Got a bit off topic here, but thanks a lot for your answers and contributions nevertheless!