Sources:
"D.(Luft) T.2110 G-2 Teil 8A, Bf 110 G-2 Flugzeughandbuch, Teil 8A Schußwaffenanlage" from 03/1943 printed 04/1943
"D.(Luft) T.2110 G-2 Teil 8C, Heft 1, Bf 110 G-2 Flugzeughandbuch, Teil 8C Sonderwaffenanlage" from 12/1943
"D.(Luft) T.2110 G-2 Teil 8A, Beiheft 1, Bf 110 G-2 Flugzeughandbuch, Teil 8A Schußwaffenanlage" from 01/1944
"D.(Luft) T.2110 G-2/R3 und G-4/R3/Wa , Bf 110 G-2/R3 und G-4/R3, Schußwaffenanlage mit MK 108 in der Rumpfspitze" from 12/1943 printed 03/1944

Cockpit of the G-series:
First I will show you the standard cockpit of the G-series, then I will go into the subtypes.

Attachment 11500

This cockpit is mostly the same as the cockpit for the late Bf 110 F with MG 151/20 instead of the MG-FF/M.
The only differences I could find is a changed right hand panel ( the oxygen-bottle is now below the panel? )
and the use of the "Schwenkplatte Sp 2 A" (the same as in the Stuka) instead of the "Sp 1 A".

Attachment 11501

The left and right front windows are also armored.
The only difference between this cockpit and the G-2/R1 with the 3,7 cm BK is the closed left ammunition-counter of the SZKK-2 (below the Revi).

The G-series cockpit with "Rüstsatz M5: 21cm BR"

Attachment 11502

No big changes here either. The plates for the power switch and the arm-switches(always on and armed) are gone for arming and disarming of the 21 cm BR.
The Button(7) for ermegency drop of the BR-tubes is changes to a switch (might be the same as in the F-series? See previous post: right side)

For the Bf-110 G with MK 108 I have found 2 cockpit variants and 5 subtype-callings.
Let's begin with the subtype-names:
First type is the original one with 4x MG 17 and then the subtype with MK 108 in the nose.
Bf 110 G-2 >> Bf 110 G-2/U9
Bf 110 G-3 >> Bf 110 G-3/U9
Bf 110 G-4 >> Bf 110 G-4/U9
Bf 110 G-2/R1 >> Bf 110 G-2/R3
Bf 110 G-2/R2 >> Bf 110 G-2/R4

135 Cartridges for the right and 120 for the left MK 108.

Now it get's a little bit tricky.
Since we have 2 pictures for the same armament dated nearly the same time (January/March 1944) but with differnent layouts of the cockpit,
I have 3 possible solutions.
First: The 1. picture I show now is a field mod allowed through the RLM with manuals, to change old type to the new subtype.
Second: Luther and GWF produced each a different version of the MK 108 subtype.
Third: Both of them are true.

By the first type with the MK 108 the "SSK 404/2" was replaced by a plate with 2 buttons and control lamps.
So nearly the same as the standard G-series.

Attachment 11503

The second type has also the "SSK 404/2" deleted, but a "SZKK-3" instead of the "SZKK-2" and only one switch to charge the MK 108.

Attachment 11504