1971-S Proof Eisenhower Dollar Prototype?

I'm not a "white metal" coin collector but see what you are saying about the difference in the reeding. The reeds on the two proof coins (two on the left in the trio) are different. And since the reeds on the coin in question seem to match a business strike example I would venture a guess that a business strike collar die was used for striking your proof coin in question.

I'm not sure how common (or rare) it might be for the collars to be interchanged like this one appears to be.

I might suggest comparing the rims of all three to previous and subsequent years to see if there might be a pattern you can see.

I'll be interested to see where this thread goes.

EDIT: You may already be familiar with the following link but thought I would post it since it may be relevant.
https://www.usmint.gov/learn/histor...hower-dollar

And the normal weight for a proof is 24.59 grams so this seems within tolerance.

Careful handling proof coins! I can see finger prints all over the fields. The smallest blemish will stand out on a proof.

@TNG, I also noticed the earths are different. The one on the left is more round instead of flat at position 8-9. I have a brown box Ike still in its package and saw that it was flat in that area also. Do you see anything strange with Mexico, the shape of the Gulf of Mexico, or anything else that is different from the other coins? The first picture is of the round earth, the next two are of the flattened area earths.

Could this be a PL 40% silver unc?

@Conder, is it possible this coin could be 90% silver? I see no copper core when looking at the reeding of the coin. It is all milky white, even when compared to the other proofs they show the core but with this one, no. Is there a way to test for silver content? EDIT: Probably a dumb question!

@Gincoin, here are the pictures requested!

Looking at the last image of the reeding, I count 20 reeds on the coin in question and 21 on the other three. (I came up with the same difference in the first image as well.)

Assuming my count is correct and there is a difference in the number of reeds, maybe an experimental collar was used to test the proper metal flow into all the devices? The link I initially posted from the mint mentions experimenting with all aspects of the minting process for the new coins from metal content, depth of relief on the dies, striking pressures, etc.

Yes there is, it just hard to see unless they tone. Silver Ike's are 40%.

Interesting about the inner core.
From Wikipedia:
"Composition- Circulation strikes:
For silver clad: Outer layers of 80% silver with a center of 20.9% silver. Aggregate 60% copper, 40% silver"

LINK
https://en.wikipedia.org/wiki/Eisenhower_dollar