I seem to remember seeing the arrestor part number for the tr5t (or maybe the MX) listed with a suffix. ie a different version. Certainly both years of the TR5T parts manual
show a much shorter version of the arrestor.
The Triumph US fitting service bulletin, 19/72, dated 12/18/72 shows a recommended gap of just 1.5" to the tightening clamp. In other words, two versions of the arrestor, one for the B50MX/TR5MX, and one for the TR5T.
Tridentman, you are correct about the fitting place. Triumph USA Bulletin 15/72 "1973 Uncrating and servicing instructions" item 19, "Install mufflers (T100:.......) TR5T: Install Spark Arrestor.
I guess what this means for me is, its ok to present a bike without one!