The square thrust block pictured is factory original and correct for pre-'64 rear ends.

Prior to '64 the center thrust block width was different for Sure-Grip and non Sure-Grip differentials; the square thrust block was about 1/4" thinner than the Sure-Grip buttons and needed a longer axle. Starting in '64 they decided to make the axles the same length and introduced the wider round thrust block for the non Sure-Grip units.