According to the Märklin book:
Old German Bundesbahn lettering:
Main lettering:
G: closed standard car, 15 tons load, flat or arch roof
GG: closed standard car, 30 tons load (or more), round roof
T: heated or refrigerated cars, 15 tons load
TT: heated or refrigerated cars, 30 tons load (or more)
K: cars with covers or sliding roof, 15 tons load
KK: cars with covers or sliding roof, 30 tons load (or more)
V: cars for small animals, 15 tons load
O: open cars, 15 tons load, 80 cm or higher walls, possible to tilt
OO: open cars, 30 tons load (or more), not possible to tilt
R: flat cars with rods, 15 tons load, minimum loadable length 9.9 m, walls (if any) removable
S: flat cars with rods, 15 tons load,loadable length 13 m, 40 cm removable walls
SS: flat cars with rods, 35 tons load, loadable length 15 m
H: flat cars with tiltable rods, 15 tons load
X: open cars for railway work, 15 tons load
XX: open cars for railway work, 25 tons load (or more)
BT: cars for big containers, 18-24 tons load
sub-lettering (applicable main lettering in parenthesis):
a (SS): open brake platform
b (G, R, T, BT): specially made for ferry transport
c (O): wooden walls, higher than 130 cm
d (K, KK): cars for powder material, unloading with compresed air
e (all): electric heating
f (T): fish transport
f (K, KK): dors at the short ends of the car
f (X, XX): flat cars
ff (O): double-stock car transport
g (G): emptied through the bottom
g (T): for deep frozen goods
g (V): four-stock cars for poultry
gg (T): for carbonic-acid snow and deep frozen goods
h (all): steam heating
hh (G, GG, T): steam heating with heating device on board the car
i (O): sideways tiltable
k (T, TT): isolated, but not for deep frozen goods
k (K, KK, G): can be loaded/unloaded with a crane
k (O, OO): with barrels
k (S): loadable length less than 13 m
k (SS): loadable length less than 15 m
l (G): loadable area more than 26 m**2, and arch roof
l (O): loadable length at least 10 m
l (SS): loadable length 18 m (except together with sub-lettering yl)
l (X): loadable length at least 8 m
ll (G): permanently coupled pair
m (G, K, O, R, S, X, BT): 20 tons load
m (SS): more than 35 tons load
mm (G, K, O, R, X): more than 20 tons load
n (T): not suitable for deep frozen goods
o (T, TT): no meat hooks
o (R): rods temporarily removed
o (X, XX): fix walls, higher than 40 cm
p (O): not tiltable
r (G, T, K, V, O, R, S): transfer cars, can be transfered to wide gauge
s (G, GG, T, TT, K, R, BT): in German domestic traffic suitable for speed up to 100 km/h
t (G, GG): doors at short ends of the car
t (O): automatic unloading through the bottom of the car, sloped floor or funnel
t (OO): automatic unloading through the bottom of the car, sloped floor or floor covers
t (X):floor covers or funnels
t (S, SS): deep loading cars
u (O): sideways tiltable, with lifting device
v (G, GG): animal transport cars with keeper's cabin
v (T): electric circulation
v (O): wooden walls, over 190 cm
w (G, T, V, O, S, H, X): loads less than 15 tons
w (GG, OO): loads less than 30 tons
w (SS): loads less than 35 tons
y (SS): loads 50 tons, load length at least 8.8 m, load width at least 2.83 m, open brake platform, tiltable platform hand rails, transferable to wide gauge
yl (SS): same as y but load length less than 8.8 m (?)
ym (SS): loads 80 tons, loadable length at least 11.2 m, open brake platform, tiltable platform hand rails, transferable to wide gauge
z (OO): suitable for ore transport