The Left hand threads on the left side goes way back to the chariots then carried forward to buggies , covered wagons, automobiles as a means of keeping the wheels on the rig.
We're talking about the axle nut on all these early vehicles. The logic was sound, It kept on to later vehicles when lug nuts and studs were used to hold the wheels onto the hub.
But as you know Chrysler was the last to use them all the way into the 70's.
Old habits die hard.
The fact worked on the axle nut until the cotter pin came along, and on knock off hubs.
But the theory falls apart on lug nuts.
As for the SBP on the early A bodies, remember they only had 9" brakes so the hub was smaller and the wheels were only 13". Not much room for a larger BP.
What I find as real odd in 73 they put single piston Disc brakes on the front of the A bodies with the LBP and if the car was a \6 it had the 7 1/4" rear axle with 9" brakes and a SBP.
And NO it wasn't just on one odd car. I saw it on a few dozen cars. But I've never seen a multi pattern wheel for the cars. Go figure.