I got an Amiga 2000, board Rev. 6 Does anyone know what is is?
It is not an aftermarket mod. As evidenced from numerous pictures on the net.
Like: A2000 Revision 6.3 Motherboard - on Big Book of Amiga Hardware – Amiga Models & Clones / Commodore: A2000
As explained there:
At the time, the Rev 4.x was in production, and the new board was desired for new production, as the price crossover from the 256K x 1 to the 256K x 4 memories had been reached. However, the Rev 5 board was completed ahead of the need for it. So the PCB guys did some additional cleanup work on the design, and I think the FCC people got involved, too. Regardless, the resulting Rev 6 motherboards were plagued with problems (in fairness, some were due to the switchover to a 68000 with faster signals, that was also noisier). - Dave Haynie
It's just a late time fix to get it shipping.
–– From: Amiga A2000_R6 Rev.1.38 (05.09.2012) AmigaWiki PDF
Some call it the "funky bridge" as explained in Amiga 2000 Rev 6 to Rev 6.2 (or 6.3) hardware changes.
It appears to be
Rev. 6.2: "Solder series connection of 68 Ohm and 270 pF to the very right of the 4 vias below CN303, other end of the circuit to ground. Eliminates power on Guru".
–– as documented here: AmigaWiki A2000 Mainboard Upgrade