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there is a group in the US for people that collect US plate number singles
Indeed there is, and they have some very helpful information on their website at this link:
http://www.apnss.org/examples.htmI find it especially helpful that they list new plate numbers on current issues, too, including some perforation varieties, as well.
Some collectors take great effort in trying to collect all of the known plate numbers for a given issue, which is yet another collecting discipline to consider.