An 8 digit item number would indicate it is rather old. If it was duped in one of the earlier exploits then you would probably see two lines with the same item number in its history. As with any of this, it's hard to tell 100% but I wouldn't worry too much if the Vintage Bill's was duped. Check the history, do what ya can and if you can not find evidence that it is duped, it probably isn't.
The origin of the vintage items that should not be vintage, Bill's included, is not widely known. Some say they showed up in some players backpacks randomly, others say they were Steam support errors.
Hello. I once tried to shark a free to play wo had a buds that was clearly given to him free of charge, and when I opened the trade I realized "This is wrong. This person clearly has no idea what this item is worth and I really should not be trying to take this one expensive item he will ever have just so I could have my first item worth over 5 keys." I then promptly told him the worth of his item and that he should never EVER trade it away for an ordinary item. He then sent me a friend request and thanked me for refraining from taking his most valuable item unfairly. Now whenever I see someone trying to shark an item from someone, I stand up for the victim and tell them what it's worth before they fully fall for the sharking. Now the person I nearly sharked is almost always on the same server as I am and now I think he is just stalking me.
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The origin of the vintage items that should not be vintage, Bill's included, is not widely known. Some say they showed up in some players backpacks randomly, others say they were Steam support errors.