I attach strap dimensions for originals I have measured over the years:

pommel quarter strap..................15 to 15 1/4 inches long (+6").
cantle quarter strap..................21 to 22 inches long (+6").
side quarter straps...................60 to 62 inches long (+3").
quarter strap safes, lined............5 7/8 inches diameter.
cincha straps.........................50 3/4 inches long (+3").
stirrup straps........................54 to 56 inches long (+14")

The dimension in parenthesis is the length of the return for the
buckle(s).

The artillery cantle quarter strap had the artillery harness dee pre-riveted to the cantle quarter strap, using the same rivet that pre-attached the saddle bag stud. When the cantle quarter strap was then attached to the saddle the assembly already included the artillery harness dee and the saddle bag stud.

Then the second rivet of the saddle bag stud would then be added to center the whole assembly, and attach the dee and stud directly to the saddle.