Eric, I think the error has been mine at the checking stage. The dimension 3/4 is the brake block at its broadest point. Ray's original drawing clearly has 3/4 but there is a dimension line (in red below) missing. 3/4 being the sum of 9/16 and 3/16, both of which are shown.
Thank you for flagging this up. I hope this helps.
Diane
