Odd South Dakota Quarters

[ClonedSIM]

I generally stop by a neighborhood convenience store twice a day, and today was no different.
Each time I go I pay with cash and let the change pile up in my pockets to go through later. After I got home today and pulled out the change and saw I had 6 new South Dakota Denver mint quarters, but I saw something odd.
Three of the coins look normal, shiny and AU with a nice sandwich visible. But the other three had no visible copper sandwich at all, just nice and silvery.
Is something like this normal, just an oddity of the minting process?

 

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[whitesid]

i have never heard or saw this.
can you take picts.

 

[ClonedSIM]

I'll get some up here later this evening. Wife is dragging me out the door to the market now...

 

[whitesid]

what is the mint mark

 

[ClonedSIM]

They're all Denver quarters.

 

[Siegfried Schlagrule]

could have been inadvertantly struck on silver proof planchets. You'll need the weight to verify that. siegfried schlagrule

 

[ClonedSIM]

I took a very close look at them, and there actually is a copper band on the side, but it's almost microscopic, and rides just below the top edge of the quarter. Imagine a sandwich with a huge piece of bread at the bottom, one piece of ham inside, and then a very thin slice of bread on top. It gives the illusion that the copper is missing, but it's just tiny and not where you'd normally see it.