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My curiosity got the better of me, so I did a bit of research...

The First License Plates Issued in the U.S. (1903): Although New York was the first
state to require automobiles have license plates (1901), these plates were made by the
individual owners (with the owner's initials) rather than state-issued plates.

The first state-issued license plates were issued in Massachusetts, beginning in 1903. The
very first plate, featuring the number "1," was issued to Frederick Tudor.
(One of his relatives still holds an active registration on the plate.)

These early Massachusetts license plates were made of iron and covered in a porcelain
enamel. The background was colored a cobalt blue and the number was in white. Along
the top of the plate, also in white, were the words: "MASS. AUTOMOBILE REGISTER."
The size of the plate was not constant; it grew wider as the plate number reached into
the tens, hundreds, and thousands.

Other states soon followed. As automobiles began to crowd the roads, it was necessary
for all states to find ways to start regulating cars, drivers, and traffic.

http://history1900s.about.com/od/1900s/qt/licenseplates.htm

On collecting old license plates:

http://www.ask.com/wiki/Vehicle_registration_plates_of_the_United_States

http://www.pl8s.com/faq.htm

One can only imagine the size of the ring binders that would be needed...