Need help identifying/explaining a part.

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I need some help from the real experts here on the SuperCar site, explaining and identifying these two parts. I know what they look like and what they probably are, L & R Rear Axle Bumper Brackets for a 68-69 Camaro or Firebird. However, they are not slotted. Instead they have a 3/8" hole drilled in place of the slot that is somewhat off centered.

While selling off the remained of the items in my shop, I came across these. I really did not notice they were not slotted until a couple of days ago. What I do know is they have been on my shelf for 15 years, they must be a factory part and part of a frame off restoration because they are powder coated. After a couple of days of searching my mind and the internet I can't find any original or aftermarket axle bumper brackets for 68-69 Camaro or Firebird that are drilled and not slotted, which I find very weird. Any help identifying these as to the correct model, year or application would be greatly appreciated.
Thanks, Rick