You are in luck I found NK1011 in the
1955 Chevrolet Truck 1st Series Restration Pack Go to pdf page 6 for rear axle numbers.
Rear Axle Numbers
NK1011
N = Year, 1955 1st series
K = Type and Assembly Plant
K = Type, 3100, 3.9 ratio
K = Assembly Plant, Detroit
10 = Month, October
11 = Day of the month, 11th
Axle was manufactured on, October 11th, 1955 at the Detroit, Michigan GM Assembly Plant,
Axle has a 3.90 ring and pinion gear ratio
Axle was manufactured to go into a 3100 1/2 ton 1955 1st series truck
Someone swapped the drive train in your 1951 3100 truck with a 1955 1st series truck 3100 drive train.
They also changed or changed out the transmission to except a open drive shaft.
1955 1st series is when Chevrolet stopped using the torque tube drive line on trucks.
