It’s an engineering sample that was produced before the final product was available. They use the really early ones to figure out if what they got back from the fab actually runs and how fast it will go safely. Later ones end up at motherboard partners so they can test their new board designs.

It is pretty common for them to leak out onto the second hand market after the final release. I’ve never heard of one that had any real problems, but in theory you might be buying something that has some issue that they hadn’t discovered at that point.