Believe these right and left metal seat back/covers are from a '67 Corvair Coupe but, I could be wrong. They're a bit dirty; few minor soft dents; and minimal corrosion. I'm willing to trade for NOS or older, made in the USA NORS items.

Some Corvair Guy in Ramona

Correct.
Metal backs for the Astro-Buckets, was a '67 model year only item.

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WA. state, 1 mile south of the Canadian border, I am not at the end of the world, but you can see it from here.

But only for the first part of the model year and/or if the car was a 4 door. After about the first 22-23,000 cars the two doors were switched to a plastic back, and ONLY the 2 doors. The 4 doors remained metal back throughout the entire model year.

-Mark

Good info there Mark.
That would be expected.
They already had more than they needed for the four-doors, and had to use them up.
Whereas the coupe and convertible seats were much more numerous, and the plastic backs were much cheaper to make.
You know The General, never spend a dime, you don't need to!

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WA. state, 1 mile south of the Canadian border, I am not at the end of the world, but you can see it from here.

Possible, Mel. But having worked for them and knowing how they work, I suspect that the more likely reason was because the impending change to the plastic back and the need for a stationary seat back. The 4 door's seats use a single bolt put in from the bottom that eliminates the more expensive latch and makes it possible and/but necessary to eliminate the slot in the back, whereas the latch requires the slot.

The Corvair's Astro bucket is used on the high-volume Camaro and Firebird, but only the Corvair used the less expensive, latch-less 4 door seat. They were stuck with the more costly latch seat, and may have found that making the back out of plastic was cheaper. But they wouldn't spend the money adding the slot into the plastic back molds, since they knew the 4 door was going away at the end of the model year,

Thus the running change to plastic was made as soon as the molds were ready, but they had to let the 4 door soldier on without the running change.

-Mark