I owned a 1967 S code 4 speed. The VIN was stamped on the bottom also. Mine was unusual in that they didn't use the "S" stamp. Whoever stamped my toploader used a "C" punch twice. Made it look like a big "S" by rotating 180° for the lower half of the "S".
FYI, I bought a HEH-BX toploader and it had a 1966 K code VIN stamped on it too.