Some brands also (besides the stamp) have different barcodes between Oregon and Washington. Pepsi and Mt dew for sure may be others that I'm not aware of. doug almost got t right. Ball the jar company makes 95% of the cans, where I'm not sure, regardless they all get stamped. Washington (because of issues with people trafficking cans across borders and collecting on cans for which no deposit was payed) requested that Ball produce cans with no stamp for them. A few companies bought in to the idea. So ball actually has the stamps sanded off. Most likely cheaper to add a sanding phase than to break down the entire production line and reassemble it with out the stamper for 2-5 million cans. But yeah, no stamp=produced for Washington. Not that it really matters that much but I'm bored.
If you look at the top of the cans and you do not see the OR 10 cents stamped in the aluminum, the cans were manufactured in Washington
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