[Scarebird]

This is what I found:

SKF 16404, NAPA NOS 16404 M and P axles (?)

Width (IN) 0.5
Shaft (IN) 1.618
Outer Diameter (IN) 2.579




BCA NS2146

Housing Bore (IN) 2.560
Shaft (IN) 1.687
Width (IN) 0.375



SKF 16735, NAPA NOS16735 B and O axles

Outer Diameter (IN) 2.556
Width (IN) 0.47
Shaft (IN) 1.688



TIMKEN 712146 8.5" ring gear

Outer Dia (IN) 2.562
Seal Width (IN) 0.470
Shaft Size (IN) 1.688



NATIONAL 2146

Housing Bore (Inch) 2.560
Inside Diameter 1.687 in.
Width (IN) 0.375




As you see there is one outlier (M or P axle = 12 bolt?). Other than that there are 2 heights, 0.375" and 0.470".

Earlier 8.2" BOP axles are smaller, so it is not that - nor is it later C-clip axles...