If GMC rear-end codes are the same as Chevrolet codes:

NK 1224
NK = 1955 1/2 ton rear-end from the Detroit assembly plant
12 = December
24 = 24th

If that is correct, it would be a 3.90:1 rear end (open drive-shaft).

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.