I found this coin metal detecting in Seattle WA on a sidewalk, so who knows the back story on how it ended up there. I really have no idea what it is.

I understand how you might be able to match up same serial number pairs of dollar bills, including with the same prefix and suffix letters, if they have fancy serial numbers, since fancy numbers are saved as collectibles. But how about matching up a pair with a random number, one that you take out of your pocket. I have 2 one dollar bills with same number C40901138A, series 1993 and 2009. I can see nothing whatsoever that is special about this number that would catch any ones attention. They are graded 30 and 35 by PMG, meaning they were not saved from new, but pulled from circulation, and around 16 years apart?

The number of one dollar bills printed for each series for all Federal Reserve Banks varies, but an average of about 10 billion is close enough. This means if you pulled a random serial number dollar bill out of your pocket, the odds of finding an exact numbers-letters match for that bill would be about 1 in 10 billion. In other words, if you looked through 1,000,000 bills your odds of finding an exact match in those one million bills would be 1 in 10,000.

I got them in an eBay auction. The seller does not know how they got matched up, and the same goes for the person he got them from. Who would ever want to spend even 10 minutes of their time looking for such a match?

Can you think of any plausible idea of how this might have happened?