C-421 was produced in 1967 and 1968 - 199 aircraft.
C-421A was produced in 1969 - 157 aircraft. GW was increased from 6800 to 6840 lbs and an increase in 75% critical altitue to 22,500 feet.
C-421B was produced intially in 1970:
- Nose section extended 25 inches
- wing span increased by 2 feet
- beefed up landing gear
- larger capacity oil cooler
- more efficient turbocharger
- cowl flaps were eliminated
- MGW was increased from 6,840 to 7,250
- Critical altitudes were increased to 18,000 feet for 100% power and 25,000 feet for 75% power. These are the maximum altitudes at which those % power can be obtained
C-421B 1972 models increased MGW to 7,450. Cabin diff increased from 4.2 to 4.4 psi.
C-421B 1973 models increased max diff from 4.4 to 5.0. Rear pressure bulkhead moved aft 7 inches. New cabin windows. Instrument panel moved forward, lowered and tilted. Switches in the side console were rearranged. Teflon coated landing gear strut bearings.
693 C-421B were produced between 1970 and 1975.
Source -
http://www.aircraftspruce.com/catalog/bvpages/13-17412.php