Ladies and Gentlemen: we got him!
Yes, I am now the owner of a genuine 60 watt Mesa Boogie Mark IIC+.
Serialnumber 13685.
Quality signature on the back (I think it's PG), the hand-drawn + on the back and it passed the loop test.
It's an SRGX 39 (Sixty Watt / Reverb / Graphic EQ / External...