First of all, where does "amber" come from? It does not look like it even with the blue flash being used. Do the covers really look like this viewed on your computer or phone vs. in real life? You need to give us something better to start on. An amber paper is a pastel-like yellow color. See:http://goscf.com/t/62678
which, scans or not, are pretty accurate if not spot on. Your exact design is shown in the poster shown by studebaker51.
So Scott U411 if on white paper, U412 if on amber paper. The indicium looks to be Die 3, so then more specifically U411b or U412b.
These fall into the broad EFO (errors, freaks, oddities) category. Printing mistakes like this (reversed, albino/no ink, upside down, double overprints, etc.) are rather common in US stamped envelopes. They might sell for $5 up as the basic envelope is extremely common and very cheap. So, something of value there but not huge rarities.
sharons25, your patience is appreciated, but if Chipshot thinks SCF is supposed to be the home of the instant answer, he's got another thing coming.