The bigger (B-body size) upper ball joints were part of the front end change for the US 1973 model year. The K-member changed at the same time. The stabilizer bar/anti-sway bar/anti-roll bar (take your pick) threads through the center of the K-member instead of around the front as it did in previous years.
Other chassis differences beginning with the US 73s were the changes of the disk brakes from 4-piston, fixed caliper to single piston, floating caliper and on the cars with disk brakes a change from the 4 inch bolt pattern to 4½ inch bolt patterns.
What I'd like to know from you RHD guys, did the A-bodies manufactured in AU and ZA have 4 or 4½ inch bolt pattern wheels? Or is it something else entirely?
FWIW: The 904 for the /6 is referred to as the 904G (G/RG engines)
The 904 for the LA is referred to as the 904A
There were also designations for the 904s in AMC V-8 and I-6, but I don't know what it is.
