It was a GR-4/GT-4 part, possibly shop-fabricated by individual race teams for Spa, LeMans etc. Those races originally required the car to carry a mounted spare. A 15" x 14" wide spare rear wheel/tire won't fit anywhere else! But with only 14 factory-racing Panteras built 50+ years ago, DeTomaso didn't build those brackets that I know of. So they probably all were a little different. Look for photos of the engine bay of the GR-4s- I know I've seen one somewhere.
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