Based on the '66, '69, and '71 K20s I owned: The Rockwell was still used in 1969, but the NP205 was used in 1971. Not sure which one was used in 1970, or if it was switched midyear. Also, the 1969 Rockwell was directly attached to the transmission. The 1966 was divorce-mounted (short driveshaft between tranmission and transfer case); again not sure about 1967 and 1968. The 250 6 cylinder made its debut in 1966. Hope this helps a little.
