5 Cent Blue Airmail Stamped Envelope, Which One?

So this is die 2 with a #30 watermark and diamonds that measure 9 mm on all sides. what is it? It has pencil written on it UC1 bordered with no watermark.

So when they say on oblique side them mean the angled side going up and down.? Then it measures11. I have die 2 , border A with Watermark 30, which was common?

Assuming everything you have reported is correct, then you have UC2, if size 10 UPSS AM8-30 valued at $20, if size 13 UPSS AM9-30 valued at $ 60. However, if it is watermark 30A with top of the "S" cut off then AM8-30A valued at $14 or AM9-30A valued at $14 respectively. Sorry, I can't tell size 10 from size 13 from the scans. Both have the same number of red and blue lozenges along each edge. Size 10 is 89 X 160 mm and size 13 is 94 X 171 mm. Both are valued at $12 in my 2018 Scott Specialized.

Thanks Bill! Yes size 10 watermark 80 not 80a. I also have scott U438 oriental buff color, both 3s slanted right (die7), high tops, but with watermark 29 and purple ink not dark violet. Scott only lists with dark violet

The long period of time for most circular dies, the method of mixing ink colors, and a few wars led to many variations in color which are not catalog listed. Only real color errors are listed and except for putting black in the ink fountain the other color errors are actually an error of denomination but color error sounds so much better!