That's the equipment managers year and size code. These are only put on game jerseys. The retail jerseys don't have them. Surefire? Well nothing is, but with the NFL Auctions you can be pretty damn sure. I have a friend who knows one of the Bills equipment managers and he told me that all the Authentic listed items are really team issued gamers.
So, the ones I have seen have to be made for the team and as all NFL on field jerseys are made by Ripon, they have to be.
Another way to tell a gamer is the Reebok vector logo on the sleeves. On retail jerseys this is embroidered directly onto the jersey, with a gamer the logo is a separate patch which is added to the sleeve, sometimes there are small differences in colour on the patch which makes it easier to see.