Smith and Wesson Revolvers are classified by size and designated with a letter. There are three basic sizes:
J Frame: .38 Special and 9mm. This is the smallest frame. Early models may have a square butt or a round butt but all later models have round butts.
K/L Frame: .38 Special and .357 Magnum. This is the medium frame revolver which may have a square butt or a round butt, but all later models (since mid 1990's) have round butts.
N Frame: .41 and .44 Magnum. This is the large frame revolver which may have a square butt or a round butt, but all later models (since mid 1990's) have round butts.
J Frame Round Butt (.38 Special and 9mm)
Models 242, 296, 31, 34, 36, 38, 317, 331, 332, 337, 342, 40, 42, 042, 43, 49, 442, 65, 631, 632, 640, 642, 651, 940
J Frame Square Butt (.38 Special)
34 ( Kit gun .22/.32 post 1960), 35 ( post 1960), 36, 37, 38, 49, 51, 60, 640, 649, 650, 651
K & L Frame Round Butt (.38 Special and .357 Magnum)
10 (post 1994), 13, 19, 30, 500, 64, 65, 66 (post 1995), 686
K & L Frame Square Butt (.38 Special and .357 Magnum)
K-32, 10 (pre 1994), 11, 13, 14, 15, 16, 17, 17 Plus, 18, 19, 31, 32, 33, 34(pre 1960), 35 (pre 1960), 48, 66, 67, 547, 581, 586, 617 Plus, 648, 681, 686
N Frame Round Butt (.41 and .44 Magnum)
29, 610, 625, 627, 629 & Magna Classic
N Frame Square Butt (.41 and .44 Magnum)
20, 21, 23, 24, 25, 26, 27, 28, 29, 520, 610, 625, 629, 657 & .44 Classic Hunter