How rare is this?

Trim tags can be reproduced so it is possible to 'fake' them.

The "tank sticker" is also known as the 'build sheet'. Most often they are found glued to the top of the gas tank but have been known to show up in other places in a car. There are many, MANY threads in this forum (and a few in other forums on the site) that discuss/lament/etc the documentation, or lack there of, available on C3s.
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Jim O.
Save the Wave...!!!

When you say you have the order form, what does it look like? That may be a copy of the build sheet. The build sheet say Corvette Order Copy at the top.