Hi gette, and welcome to the forum. Your post is a bit unclear. How do you know it is a U320? I suppose you can not post a picture - if not can you give a description of this stamped envelope? By the way, ik ben zelf een Nederlander, wonende vlakbij Atlanta, Georgia.
hello. it has come to my attention that @stamperix might be right, however my red stamp kind of does not look like the other ones, so I will have your judgement. I now believe that him is not a good one
Gette, the design number of your cut-out seems to be U79. Now, depending on the color of the envelope the catalog number would be something between U365 and U370. Hard to tell what the paper color is on a computer screen without some other, known color. The deign looks like it is die 2.