S&W Revolver Frame Sizes:

M frame: Early Ladysmiths, discontinued before model numbering.

I frame: Predecessor to the J frame with leaf mainspring.

Improved I frame: Successor to the I frame with coil main spring.

J frame.

JAS frame: Shrouded hammer with alloy frame.

JS frame: Shrouded hammer with carbon steel frame.

JA frame: Alloy frame w/fixed sights.

JAT frame: Alloy frame w/target sights.

JC frame: Fully concealed hammer.

JAC frame: Fully concealed hammer w/alloy frame.

JR frame: Carbon steel J frame for .38 S&W cartridge.

JT frame: Carbon steel frame w/target sights.

JTM frame: Carbon steel frame w/target sights for .22 Magnum cartridge.

J Magnum frame: For .357 cartridge, alloy or steel, w/fixed or target sights.

C frame: Between J & K frame size, used only for the non-cataloged Model 73 in carbon steel.

K frame: Carbon steel frame w/fixed sights.

KA frame: Alloy frame w/fixed sights.

KAT frame: Alloy frame w/target sights.

KR frame: Carbon steel frame for the Model 11.

KT frame: Carbon steel frame w/target sights.

KTC frame: Carbon steel frame for the Model 53.

KTM frame: Carbon steel frame for the Model 48.

L frame: Carbon steel frame w/fixed sights.

LT frame: Carbon steel frame w/target sights.

N frame: Carbon steel frame w/fixed sights.

NAT frame: Alloy frame w/target sights.

NA frame: Alloy frame w/ fixed sights.

NT frame: Carbon steel frame w/target sights.

NTM frame: Carbon steel frame w/target sights for the Model 28.

E frame: Stainless steel for the Model 60.

ET frame: Stainless steel frame w/ target sights.

F frame: Stainless steel K frame for the Model 64.

FT frame: Stainless steel frame w/target sights.

H frame: Stainless steel L frame for the Model 681.

HT frame: Stainless steel L frame w/target sights for the Model 686.

G frame: Stainless steel N frame.

X frame: Stainless steel for the .460 & .500 S&W Magnums.

Source: Standard Catalog of Smith & Wesson, 3rd Edition, by Supica & Nahas.