OK ! I've got a head-ache now !
The discoloration you see on the diode board might be simply some corrosion, or could be the result of overheating. Things do get pretty hot there anyways, but if a diode has gone bad usually the back metal surface (the 2nd photo) will show a little "bronze" coloring of the normally silvery diode backing. We probably ought not to try to get into testing the individual diodes, but here is some more info for you.
BMW did issue a service bulletin for pre-81 bikes on Sept, 1988 (#12-012-99 (2323) according to my Clymer manual) for warranty replacement of the diode board if the
model had a black electrostatic dip paint finish on the timing cover. The symptoms identified were melted solder joints on the board, low battery charge rate and/or partially glowing GEN light. But, we may as well check that perhaps we can "fix" the problem (if it isn't bad solder joints and is simply bad grounding at the diode board) by cleaning up any corrosion that you see on all the mounting/grounding points and tabs where the wire plug onto the diode board. Check that all wires that plug into the board are not pinched/kinked or have worn-through insulation, loose fitting contacts etc. Clean all these contacts with some emery cloth or electronics contact cleaner and an old toothbrush. Make sure things are dry before you reconnect and try to fire the ol bike up.
As for the alternator parts - yes indeed, those brushes should be fine at that length. If your volt/ohmmeter has a low resistance scale, switch it to that scale (it might say something like "200 Ohms Max" or similar, or maybe not. In any case, we are going to be measuring some very low (small) resistances when checking the alternator, and we want to make as accurate a measurement as we can. More expensive meters will simply autorange/select the lowest/best display range - not sure what you have for a meter, but anyways....
The resistance measured on the rotor between the slip rings (brushes not touching rings) should be between about 3.0 and 3.8 ohms. If you measure more than 4 or 5 ohms, or less than 2.5 ohms, then there may be a problem with the rotor.
The resistance of the stator windings should be more than 0.5 ohm and less than about 1 ohm if your bike is at least a 1977 model (which it is). This is measured at the multi blade electrical connector that is wired to the stator coil windings.
If these resistances all check out OK, I'd clean up any/all terminals that wires plug into, clean the slip rings with emery cloth and electronic contact cleaner or isopropyl alcohol, dry and reassemble. If after doing this and cleaning up any corrosion, etc on the diode board, put all the electrical system back in place and start the engine.
If you still aren't getting proper charging voltage, there is either a bad wire connection somewhere or the diode board is indeed in need of replacing, I think. It seems
that we'd already eliminated the voltage regulator from the list of problem suspects, and we know that your GEN light bulb is working... others may have other opinions/input to help you out..
Best of luck ! Don't despair - you can very likely fix this, and you'll be very proud when you do !
Also, if you are not already familiar with the internet-wide famous "snowbum", he has a website with very good tech info on all things about these bikes, though his writing style is quite wordy and rambling, there are some 'gems' in there.. Here is a link to alternator/charging system troubleshooting on his site:
http://bmwmotorcycletech.info/trbleshootALT.htm
