Based on the reply from the BEP store it sounds as though the label is something that was applied at the BEP. If this is the case I would expect it to have been done in QC not at the BEP store. You may want to post this over on the Collectors Universe Forum to get their opinion.
I would say buy it and then see if you can have it verified as an error by a tpg. Maybe an automated machine picked up on some form of error with one of the less obvious anti counterfitting measures? Is the security thread in tact? Put the note under a black light and compare it to the other notes.
Given the limited information that we have, this is one instance where I would buy first and ask questions later. I don't see much downside (you will never have less than $160.00) and the upside could be sizable if there is an error. Even if there is not an error, there may still be some upside if the stamp is proven to have been applied by the BEP. I would buy it. Post it on the Collectors Universe forum where lots of error collectors and others hang out and see where it goes.
You really have to find someone who knows for certain what the BEP processes are. All we are really doing here is guessing because none of us know for sure. While I know the BEP has used the crayon system that others have described, maybe this is how they mark the sheets that are designed to be sold as sheets. (We can't assume that the stamp relates only to the note that the stamp is on, it may be the entire sheet). Whether the TPG personnel have this level of knowledge as to the BEP process is an open question. If the BEP has changed their process recently, the BEP may be the only people who currently know about it. Customer service at the BEP store is probably not going to know the answer. When I was there last summer, the people working in the store could only answer the most rudimentary questions about the printing process. The people who gave the tour had encyclopedic knowledge but even they may not know this level or not be willing or able to divulge it. Some of the more high powered error collectors may know the answer. This is why I keep suggesting this be posted on the Collectors Universe discussion board. Someone there should either know the answer or know someone who does or know someone who can definitively find out.