[Mermet]

Rüstsatz 3 (R3) was standard on Bf 109 G-10, G-14 and K-4. There was no reason to add a reference to it on those versions. The compressed air was taken directly on the compressor spirale.
Bf 109 G-6/U3 was the earlier designation of the Bf 109 G-8. From a letter from Mtt-GmbH, there was no specific designation of the MW 50 for the Bf 109 G-6 : that was simply Bf 109 G-6/MW.
That was not the case for the G-8. MW50-equipped G-8s were designated G-8/U3.
U4 was generally the axial MK 108 (but also the u/w MK 108). Only WNF produced Bf 109 Gs with the axial MK 108.
There was no "U1" (GM-1) on Bf 109 G-14, G-10 and K-4. Only Erla-Leipzig produced Bf 109 Gs with GM-1 device (only propelled via compressed air contained in bottles). The sole foreseen version with both GM-1 and MW50 was the Bf 109 H-2/H-3.
The MW 50 could be replaced (70 ltr) by fuel (Bf 109 K-4).
It seems that the BSK 16 in the left wing of the K-4 was Rüstsatz 8 (R8).
One Erla-built G-10 is known to have been equipped with MG 151/20 in gondolas on underwing. The designation was surely not R6, then R6 on this plane was the PSK 12 for all weather flying.

JCM