Yes, I agree, 2 inch for a Boss.
They would be normally found on 1969-70 9 inch rear ends from Mustang or Cougars. Backing plates have stamped numbers on them identifying right or left, and the various sizes. Two different suppliers, Bendix and Kelsey-Hayes.
Very likely found on other Ford products with a 9 inch rear. Watch out, in the early 1970's they changed design and those won't be compatible.
Below is a cut and paste from Dan at Chocostang
K/H Plates 65-68 Dates are not perfect for 1 3/4"---Numbers are.
70867
70866
K/H for 2"
71086
71087
Bendix Plates 69 and newer for 1 3/4"
319480
319481
Bendix for 2"
317558
317559