Statistically speaking, on modern US notes there are 600,000 different serial numbers that have 666 in some location on the note. Assuming 00000000-99999999 get released (which is usually not the case), you have a 0.6% chance of running across a modern US bill with 666 somewhere on the note.
I have a Canadian 1954 $2 with the serial number beginning 666. The seller didn't notice it when I bought it, and I didn't notice it until I looked through new purchases. The eeriest of the eeriest!
First, welcome to the neighborhood! I don't collect paper money, but that seems like it would be an interesting number to keep. Chris
I've found silver certs in circulation, but I worked in cash handling jobs in the 80's and 90's and that's when I got them. Also a nice $5 red seal 31 years ago that I still have, might only be worth 7-8 dollars, but there's a sentimental factor about pulling something out of circulation.
They're supposed to have eight numbers on them so it might be. Make sure you hold it by the edges so you don't get fingerprints on it.