Seriously, find a buyer who is far less pedantic. You don’t deserve the hassle over a $300 item. I’ve owned many examples of HD650 over almost 20 years, and yes there are slight variations but all are wonderful. 2011-ish is indeed a good year for them (I bought some HD650 drivers that year, I think); state you think that’s when they’re from and that’s all you need to do. I’m a headphone extreme guy and even I wouldn’t deal with more than that lol.
head-fi.com is the forum you seek for esoteric headphone knowledge, and they also have a classified section that’s free and very likely to snag a quick sale.