Stamp (Counter-Stamp?) on a $20 Bill

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I've seen posts on other forums about stamps on U.S. currency and I found it interesting that I found one on a Canadian $20 today. I've seen the "Where's Willy" and "Track This Bill- Where's George" stamps but this is the first one I've seen of this type.

There's speculation that these stamps may be similar to the chop marks (put on coinage by money changers)  to verify the authenticity of the bill or some stamps could be from drug dealers who mark the money to identify bills they use in transactions.

Or maybe, someone got a stamping set for their birthday.

On two separate occasions, I saw $50 notes with similar small round counterstamps, though the design inside the circle was different.  The first time, I didn't give it any attention.  The second time, I thought it was interesting but still failed to save the note because I didn't think anyone would bring it up at the forum 

This one went back into circulation the same day 

I found a $50 bill last year with the same stamp and showed it to a few dealers who were unimpressed. I quickly spent it.