The small drum spindle (shown on left) for drum applications up to about 1969 and also used on 65-67 Mustangs with Kelsey Hays 4 piston disks. A drum spindle is easily recognize by having 4 bolt holes (instead of 3 for disks) to mount the backplate. The small spindle uses A2 and A6 bearings with Nut size of 3/4 inch.
The larger drum spindle (shown on right) from 1970+ also has the same 4 bolt backplate pattern as used on the earlier small spindles but now has a larger spindle diameter and uses A12 and A13 bearings with nut size of 13/16 inch.
Both spindles types (with 4 holes) are interchangable and do well for the street but for racing application or just to beef up the early Mustangs front end a popular upgrade is to install the larger spindles. The larger drum spindles found in any 1970+ drum application bolt right up to an early 65-67 Mustang with Kelsey Hayes disks. Only mod required is to replace the early Mustang small outer Tie Rods with the larger tapered stud Tie rods found on later 70+ Mustangs.