The two mounting points with the metal "washer" are to be grounded, they are the top ones. You want a very good connections to avoid any leakage currents. There is no secret here. However, Rob Frankham strongly suggests for older airheads to run a ground wire from the battery to the diodeboard directly to give it the best possible ground (use this link). BMW had this change introduced from 1981 on I believe, but I don't know for sure.
Second the heat transfer (rubber vs. solid mounts):
That small blue (nearly invisible) area represents the area where conductive heat transfer can occur (brass rivet and solid spacer contacting), lets assume it is somewhere in the area of 0.02sqin. However, there will be a significant heat transfer loss between the aluminum body of the diodeboard to the steel spacer (low temp coefficient) to the brass rivet. I bet you, you will find the brass rivet comparably cold. I would have to dig deep to find the calculations for heat transfer, but my guts tell me that more heat is radiated though the diodeboard cooling surfaces (as intended) and that (a lot) less than 5% conductive heat transfer occurs when using solid diodeboard mounts.
I still use the old rubber shock mounts, on all my bikes, for the apparent reason: to prevent excessive vibration and shock damaging the diode internals. I can imagine, that BMW and their engineers made a conscious (financial) decision when they went from rubber mounts ($2 a piece) to aluminum spacers ($0.10 a piece). And, apart form production cost being reduced, they would be selling more of those expensive diodeboards in their Parts business. It's all about reducing cost, usually at the expense of either reliability or quality, there is always a trade-off to be made.