I checked my Magazine stash and this is what I found.
"Early in the goings. Ford developed independent rear
suspension for the Mustang using three test muels: Nos. 5F07F100023, 5F07F100024, AND 5F07F100031. This development program was later cancelled, but at least one of these test musles survives in the Pacific Northwest; the VIN is unknown."
If I had to guess I would say that these were coups and certainly pre '67 cars. The only Shelbys I saw any mention of with the IRS were on some of the competition GT-350's like SFM5R001 and from the looks of the picture in the article this is a pre '67 car.
W.