I decided to declare this group puzzle as 'almost' finished now and moved it to Solveds.
However I leave that final gap open for someone in the future who can solve this last mystery.
I cannot tell if my source is reliable in that case or not, so let's close it down for now.
The score:
Wendax: (impressive) 12 points
Tom_I: 4 points
Otto Puzzell, Oguerrerob, ftg3plus4: 1 point each