Z = Flight Information Region (FIR) Zürich with telex (back then)
G = Flight Information Region (FIR) Geneva with telex (back then)
P = Flight Information Region (FIR) Zürich without telex (back then)
T = Flight Information Region (FIR) Geneva without telex (back then)
M = Military
X = Heliport
Some examples:
LSZH = Zürich International Airport
LSZX = Schänis Private Airfield (but with telex back then!)
LSGG = Geneva International Airport
LSGK = Saanen Private Airport
LSME = Emmen Military Airfield
LSMM = Meiringen Military Airfield
L = Southern Europe
S = Switzerland
Z = Flight Information Region (FIR) Zürich with telex (back then) G = Flight Information Region (FIR) Geneva with telex (back then) P = Flight Information Region (FIR) Zürich without telex (back then) T = Flight Information Region (FIR) Geneva without telex (back then) M = Military X = Heliport
Some examples:
LSZH = Zürich International Airport LSZX = Schänis Private Airfield (but with telex back then!)
LSGG = Geneva International Airport LSGK = Saanen Private Airport
LSME = Emmen Military Airfield LSMM = Meiringen Military Airfield
LSPV = Wangen-Lachen Private Airfield LSPN = Triengen Private Airfield
LSTR = Montricher Private Airfield
LSXS = Schindellegi Private Heliport
An interesting case is Bâle-Mulhouse, a binational French-Swiss airport located in France:
It's primary ICAO code is LFSB (F = France), however, for AFTN, there's also LSZM.
AFTN = Aeronautical Fixed Telecommunication Network.