the first NAPCO's were produced in the early 50's... I have heard differring opinions on when the very first ones were produced but in general say 1951 or later... all of these early ones were either 3/4 ton or 1 ton models as the 1/2 tons during those years had a closed driveshaft which hindered installing a tcase. In 1955 chevy went to open driveshafts which allowed the 1/2 ton units to be converted to 4x4
the first 1/2 ton models were late 54/early 55... these are really rare as they are specific to AD design models and only made approx 1/2 year. In 2nd half of 1955 they changed body styles... you could still get a 1/2 ton 4x4 but these were different than the 55.1 conversions as the spring perch dimensions changed and they are not interchangeable. The 55.2 TF 1/2 ton napco's are also rare as they only made these for a half year also before switching to a new design in 1956. The 1/2 ton systems are different from the 3/4 and 1 ton systems of the same period
All of the early napco conversions had a drivers side differential and used the rockwell 2 lever tcase (not spicer 24)
In 1956 the whole napco lineup was changed to drivers side pumpkin and spicer 23 tcase... these were continued until 1959... in 1960 GM made their own corporate 4x4 system and dropped the napco system.
