OK, well I answered my own stupid question by doing a little more detective work.
Turns out, the copy of the 1971 BSA parts book I downloaded has the WRONG part number for the FBS heat shield.
My copy has part # 70-9716 listed as the correct heat shield. It's NOT ! The correct number I found out is: 71-2790
So what's the difference ???
The 70 and 71 shields look the same. BUT there's a difference in the location of the aft mounting point that attaches at the "H" connector... by about 1".
I got to looking at internet photos of various FB Scramblers and where I could zoom in, I looked at the mounting points.
The front mount doesn't change, but the rear spot got moved forward two spaces.
Apparently I'm not the only one to run into this problem, as I saw at least one bike that had an extra gizmo added to make the '70 shield fit on a '71 bike.
It's not real noticeable, unless you know what you're looking at.
Unless I can unload the '70 and find the correct '71 shield, I'll probably do the same thing.
You can all chuckle now