1951 big trucks still used Huck. Small trucks had Bendix. Has the two slots by the wheel cylinder. Read it in some of the old tech spec stuff, and then checked on my truck for sure. I haven't taken the wheels and hubs apart yet, but the flange on the brake/axle side of the dual wheel spacer has six bolts, and the wheel side has 5 studs with a ring that covers up the other five holes of the wheel as it has ten holes. Since the brake/axle side had 6 studs I assumed they might have used the same front axle with the dual spacer converting to the 5/10 wheel interface.
The other thread that I was referring to the individual had installed P30 disc brake spindles on his 51 axle with sleeves in the kingpin. If I remember the write up correctly GM didn't change the overall design of their solid axle other than widths and king pin diameters until something like 1978.
All I can definitively tell ya is my fronts are Huck though
