
When it was created on a web-fed press, where it can only be a color-omitted error.
This U550 was made on the VH-1 press, which was in use from 1965 until Ashton-Potter took over as envelope contractor in 2008.
In the early days of the 20th century there were two other web-fed presses were briefly in use: the Kenny press and the Smithe press.
Color-omitted errors are considered "major errors", whereas albinos usually have the adjective 'lowly' pre-pended, and a valuation to go with it.