I've seen an awful lot of them with the flat side pointing toward the drivers side that being said I'll bet it was at the discretion of the assembly line worker.
As for why the flat spot is there I asked my dad who worked for 38 years (1956 to 1994[I think])in Chrysler stamping plants and he said the flat spot may have been a way to save on metal by staggering the pieces being cut
Every so often I will ask him about how things were made back then, Once he told me they task him with updating the stamping die for the "new" added emission nipples on the A body fuel tank which would of been 70 - 71
