U45 is a 40c black and red entire on buff or on salmon buff diagonally laid paper and occurs only as a large envelope (105 X 249 mm) or as a cut square. Watermark is # 1, POD over US. Forgeries of the cut square exist.
The numbers you mention are years and should be related to manufacture dates of the envelope. Scott assigns 1861 as the original manufacture date of U34 through U45 with U36, the 3c letter sheet, withdrawn from sale in 1864.
I also have Scott U164 with a W2 watermark.Is that common? No star. they also have a cream down for Scott U165. And I don't have a color sample to compare for that. I have the color chart you made a while back for this series.