Sub-Varieties Of First US Stamped Envelope

Is there anyone out there who knows the sub-varieties of the first U.S. stamped envelopes? I believe this is U10 but the straight left end of the upper label made me wonder. Can anyone pin this guy down to a specific sub-variety?

U9 or U10 is my assessment.

The die types with straight ends to the labels have a colorless line across the label. Your image shows a straightish left end, but there is no line.

All of the label ends must match, both upper and lower, left and right. The label ends, label around "THREE" measurement, and the loops on each side help identify the die. Die 1 (U1/U2) has curved ends, label is 13 mm, there 10 loops on the left and 9 loops on the right. Die 2 (U3/U4) has straight ends, Die 3 (U5/U6) has K ends, label is 16 mm on both and both have 8 1/2 loops left, 9 loops right. Die 4 (U7/U8)has straight ends but a very wide, 20 mm label with 7 loops left and 7 loops right. Die 5 (U9/U10), the most common, has curved ends like Die 1 but a 16 mm wide label with 9 loops left and 8 1/2 loops right. White paper envelopes are off numbers (U1, U3, U5, U7, U9). Any paper color not white is considered buff for these issues and has an even number.