K2Fs all have a main earth brush on the top, at the drive end under the fat-headed screw, so the one on the back of the cb plate is an auxiliary one.
It is something of a hangover from the days when the same design of cb backplate was used on mags which did NOT have a separate earth brush on the body, when it was essential.
So you don't NEED it, no.
It's not a bad thing to have though, because I think it may help reduce the 'dimpling' that occurs on the brass end cheek of the armature under the main earth brush, which can cause trouble over very high mileages. That occurs at the places the brush is at, two per rev 180° apart, when the plugs fires (as it's the path for the return HT current to the armature, which is insulated from the body of the mag by paper insulating cups on the bearing outer races).
Observation over the years suggests that the depth of the dimple is perhaps less on mags that have run with the extra brush, but so much depends on mileage and usage, I can't be sure. In theory though, the 2nd brush does some of the work - maybe!
There is no brass contact ring for the auxiliary brush to run on, nope. Some cb end housings have a very rough-cast finish inside, some are finished rather better. I'm guessing that the ones with the smooth finish had earth brushes on the cb from new, while the others may not have had, but that is just a guess. CB units come with, and without.
Before consigning any camring to the junk box (unless it is obviously rusty, pitted and generally ruined) . . . it is worth doing some checks. At the fatter end of £100 a shot with taxes and transport, it's worth it.
Measure the thing with a good measuring stick at mid point on the two lobes (and in the dips for the dwell too for that matter) - if they're the same thickness give or take, but the points aren't opening on one side or the gap is very different, then either the ring is way out-of-round, in which unlikely case it'll be hard to get it into position, or - more likely - it's off-centre owing to breakdown of the insulating cup behind the outer race of the cb end bearing. Or there is a problem with the contact breaker assembly. Or there is so much wear in the housing that the ring is slopping around more thous of an inch than the points gap. Sometimes there ie even slop in the fit of the camring housing on the mag, specially if we're in mix 'n match territory. Small errors can add up, so often there is more than one thing to resolve.
Assuming things CAN be resolved, it's seriously sensible to check the timing on both cylinders. Variations of several degrees aren't unusual. A dremel used with extreme care, or a more sophisticated grinder set-up, can be used to remove a little metal from the on-ramp of the side that is opening early. Tedious, as it's trial and error, but
often worth doing.
If you can get close to equal points opening, close to identical timing on both sides, then you're there, until the next time at least! Might be good to put up a good pic of your camring so we can see what's what a bit better?
One of the beauties of the Thorspark approach is you have a wasted spark so you get even timing between cylinders - and as long as you don't leave the lights on, even a fairly flat battery will summon up the quarter or half amp needed.
