Yes! When You look closer, that plate, where 907 RR is fitted, is insulated from frame with 3 rubber holders and that RR cover(fins) are not connected DIRECT to earth ! Surely RR is conncted to earth with that minus(-) wire, but what happens, if that fails, or some bad connection in that wire???? I can quess.... I have read long time ago, that someone missed his RR , when that - wire went broken and immediatelly i made that extra wire from fins to good earth. Anyway that is "safety thing"...