[Singleton]

Originally Posted by 'ol Pinion head

I've always shined a flashlight at the center housing as well as up between the brake backing plate & the lower control arm perch. Only one finger would fit, bingo, early style A-body rear. Two fingers fit, a wider style.

I’ve always heard about the 1 finger, 2 finger way of measuring, but I feel like it’s not always accurate. Depending on the style of backing plate, it could easily fool someone as the pics below illustrate.
The black painted rear is a 64, the rusty one is a 66. Because the 66 has an extra flange on the inside of the backing plate, it has a 1” space between the lower control arm perch and the backing plate, same as the 64 with a smooth backing plate. Someone could assume that, if looking up from the ground, that both rears were the same width, (narrow). But the 66 rear is actually 1” wider. In the other pic both backing plates are aligned together on one side and there is 1” difference in width on the other.