Smith & Wesson and Colt use letter designation for various guns. Others don't. These designations are normally used only by the factory for internal control, but some were used in the formal model name (like K-38) or otherwise become widely known to the general gun-buying public.
Here are some:
S&W
J Small revolver
K Medium size revolver
L Larger revolver, but still in medium class
N Large frame
Colt
P SAA
G Peacemaker and New Frontier 22
K Frontier Scout
Q Frontier Scout and Buntline Scout
F Black powder (modern production)
D Small frame
I Python
J Medium frame (Trooper, Lawman, etc.)
SF-VI New model Detective Special
AA King Cobra
MM Anaconda
O Government Model auto pistol
S Post war Woodsman
Jim