Want to add the codes used by the Luftwaffe. Unlike fighters, other unit types used codes on their machines to identify the unit to which they belonged. Here is a list from this site.
The Luftwaffe in Scale: Identification Codes of Luftwaffe Units 1939 - 1945
Code: Unit:
A1+ = KG 53 (Kampfgeschwader)
A2+ = I./ZG 52, (I./52nd Zerstroergeschwader, from 6.7.1940 onwards: II./ZG 2 = (II./2nd Zerstroergeschwader), Flugbereitschaft Kd.Gen.d.dt.L in
Finnland (HQ German Airforce Finland)
A3+ = KG 200 (200 Special Duties Wing), Versuchskdo. 200 (200 Trials
Unit), Versuchskdo. 36 (36 Trial Unit)
A4+ = with squadron letter .A - Luftbeobachterstaffel - 3rd fighter division,
spring / summer 1944
A5+ = 1./St.G.1 (I./1st Dive Bomber Wing), SG 1 (1st Assault Wing)
A6+ = Aufkl.Gr.120 (120 Reconnaissance Group)
A8+ = with the letters .H+.K seen on a He 111 in february 1942 at Grosetto (arial torpedo trials)
B1+ = Transport Squadron of I. Air Corps (occasionally also called Transport Squadron "Don")
B3+ = KG 54 (54 Bomber Wing)
B4+ = Night Fighter Squadron Finland
B7+ = Westa 1 (1rd Metrological Squadron)
C1+ = Test Centre Peenemünde (Me 163)
C2+ = Aufkl. Gr.41 (41st Reconaissance Group)
C3+ = Transport Squadron II. Air Corps
C5+ = Westa 26 (26th Metrological Squadron)/Lfl.6 with Squadron Letter . H
C6+ = KGr.z.b.V.600 (600 Special Duties Battle Group), Tr.FI.St.4 (4th Transport Squadron)
C8+ = I.+II./KG.z.b.V.323, TG 5 (I.+II./323 Special Duties Battle Group - 5 Transport Wing)
C9+ = NJG 5 (Nachtjagdgeschwader) also NJ 100 (100th Night Fighter Wing)
D1+ = SAGr.126 (126 Maritime Reconnaissance Wing), LD.Kdo.65 (65th Courier Detachment)
D3+ = 2nd Night Harassment Group
D5+ = NJG 3 (3rd Night Fighter Wing), frequently also with the code L1+
D7+ = Wekusta 5 (5th Metrological Squadron; from April 1943 onwards in connection with the code letter "N", previously 1B+ with the squadron letter "H")
D9+ = Night Fighter Squadron Norway, temporarily I./NJG 7
E1 - E7+ = Test Centre Rechlin (for Me 163 and Ar 234)
E8+ = 1./FKG 50
F1+ = KG 76 (76th Bomber Wing), I./St.G. 76 (I./76th Dive Bomber Wing; using the letter C,M,N,P of the II. Group as those of the I. Group had already been allocated to the I./KG 76 and, from 6.7.1940 onwards, to the III./St.G. 77)
F2+ = Erg.(F)Gr. (Long-range Reconaissance Replacment-Group)
F3+ = Aufkl.Gr.12 (12th Reconaissance Group)
F5+ = NAG 5 (5th Short-range Reconaissance Wing)
F6+ = Aufkl. Gr.122 (122th Reconaissance Group)
F7+ = I./LLG 2 (I./2nd Glider Assault Wing), SAGr. 130 (130 Maritime Reconaissance Wing; frequently also with the code 6I+)
F8+ = KG 40 (40 Bomber Wing), Transport Squadron "Condor"
G1+ = KG 55 (55 Bomber Wing)
G2+ = Aufkl.Gr.124 (124th Reconaissance Group)
G3+ = NJG 101 (101st Night Fighter Wing)
G5+ = Transport Squadron V. Air Corps
G6+ = KGrzbV.2; 101, 102, 103, 104+105 (2nd, 101st, 102nd, 103rd, 104th+105th Special Duties Battle Group), TG 4 (4th Transport Group), 1st Air Transport Group (Maritime)
G8+ = Transport Squadron IV. Air Corps
G9+ = NJG 1 (1st Night Fighter Wing), ZG 1 (1st Heavy Fighter Wing; initial formation), NJG 4 (4th Night Fighter Wing; only for a short period of time)
H1+ = Aufkl. Gr.12 (12th Reconaissance Group)
H4+ = LLG 1 (1st Glider Assault Wing)
H7+ = St.G.3 (3rd Dive Bomber Wing), SG 3 (3rd Assault Wing)
H8+ = Aufkl. Gr.33 (33rd Reconaissance Group)
H9+ = 7th Air Transport Squadron (Maritime)
J2+ = NAG 3 (3rd Short-range Reconnaissance Wing)
J4+ = 5th and 290th Air Transport Squadron
J6+ = KGr.z.b.V 500 (500 Special Duties Battle Group)
J9+ = StG 5 (5th Dive Bomber Wing), SG 5 (5th Assault Wing) (previously l./186fh Dive Bomber Wing, carrier-based, from 6.7.1940 onward III. /Dive Bomber Wing 1)
K1+ = Messenger Squadron of Ob.d.L. (High Command of Luftwaffe)
K6+ = Kü.fl.Gr.406 (406th Coastal Aviation Group) KG 6 (6th Bomber Wing).
K7+ = Aufkl.Gr.Nacht (Night Reconnaisance Group)
K9+ = Aufkl.Gr.B.d.L. (Reconnaisance Group High Command Luftwaffe) (also T5+)
L1+ = I./NJG 3(l./3rd Night Fighter Wing), I. u. IV. (Z)/LG 1 I./IV. (Heavy Fighter)/1st Instruction Wing, also I./5th Dive Bomber Wing (former IV./1st Dive Bomber Instruction Wing)
L2+ = LG 2 (2nd Instruction Wing)
L5+ = KGr.z.b.V.5 (5 Special Duties Battle Group), 7./Erg. Transportgeschwader (7./Transport Training Replacement Wing)
L7+ = seen with +AB-AF on Fi 156, Me 108 and Me 110
M2+ = Kü.fl. Gr.106 (106th Coastal Aviation Group)
M3+ = seen on a Ju 52/3m (unit ?)
M7+ = Kü.fl. G r.806 (806th Coastal Aviation Group)
M8+ = ZG 76 (76th Heavy Fighter Wing - first activation)
N1+ = High Capacity Transport Squadron from 30.1.1945 onward
N3+ = I./KGr.z.b.V.172 (I./172nd Special Duties Battle Group also 4V+)
N6+ = Unit Major Babekuhl
N7+ = seen on a Do 18 (unit ?)
N9+ = Flugbereitschaft Norwegen (Air Service Norway)
P1+ = KGr.60 (60th Battle Group)
P2+ = Aufkl. Gr.21 (21st Reconnaissance Group)
P4+ = Führungskette X. Fliegerkorps (Staff Flight X. Air Corps), Transportstaffel Fliegerführer Nord (Ost) (Transport Squadron Air Commander North (East)
P5+ = Sonderstaffel Trans-Ozean (Special Squadron Transocean), KGr.zbV. 108 (108th Special Duties Battle Group), Kdo. d. Transportfl. Chef See-Norwegen (Command of Chief. Maritim Transport Norway)
Q5+ = Westa 27 (27th Metrological Squadron)from October 1943 on, previously 5M+ together with Westa 26 (26th Metrological Squadron)/Lfl. 2 as an Squadron in Southern Greece
R4+ = 2 Night Fighter Wing
S1+ = St.G. 3 (3 Dive Bomber Wing (also S7+)
S2+ = St.G. 77, SG 77 (77 Dive Bomber Wing) (77 Assault Wing)
S3+ = KGr.z.b.V.5, Transportgruppe 30 (5th Special Duty Battle Group, 30th Transport Group)
S4+ = Kü.fl.Gr.506 (506th Coastal Aviation Group)
S7+ = St.G.3, SG 3 (3 Dive Bomber Wing, 3 Assault Wing)
S9+ = Erp.Gr.210, (at first 210th Testing Group until April 1941, then 210th Special Battle Group and from 1942 on Heavy Fighter Wing 1 together with 2N+)