The 67-72 model C30's were available with 16" dual wheels and they used the same axle except they look like this http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1146.jpg instead of this http://i305.photobucket.com/albums/nn214/jfnar/57%20chevy%203800/DSCF1147.jpg
I assume that the extra spacer built into the hub is to move the wheel outward to provide clearance for the inner 16" dual wheel. If I remember correctly it's about 3/4" thick.


'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc