Since Groily has opened the topic of U.S. registrations, this might be interesting to some.
When I bought my bike, it was registered in California by the engine number, only. When I was incredibly stupid (2003, I like to think more stupid than I am now), I dumped my crashed original frame for a frame bought on eBay. In stripping the paint from the eBay frame, I noticed one number on the frame was stamped over another. EEEE! I called the Illinois vehicle department (for short, let's call it the "DMV") and spoke with the one lady who deals with awkward title situations such as this. I didn't want to proceed any further with a frame (and the rest of the parts) that could be confiscated out from under me. She offered to send DMV police to view the frame and make a judgment. Well, it turned out that, instead of DMV police showing up, it was two officers from the Illinois State Police, not in uniform. They were both riders and appreciated the project in the works. They viewed and recorded the frame number(s) and it was their opinion that it was probably a factory overstamping. I assume they couldn't really tell, but I was happy with their conclusion. The DMV got back to me advising that there was no record of the number(s) being associated with a stolen bike, so I was clear to register it. When it all came down, all I did was walk into the DMV with the California title and they issued a new Illinois title based, again, on the engine number only.
It may be amusing to learn, if you didn't know already know, that the previous Illinois governor at the time was sent to jail for, among a number of other things, selling truck-driver licenses through the DMV.
Richard L.