Canada Scott/Unitrade 789 Light And Dark Cameo

There is a noticeable difference in the depth of shading in the Queen's
cameo when comparing Plate # 1 and Plate # 2

Plate 1 is light grey whereas Plate 2 is dark grey
The green background is also lighter and brighter on Plate 1





Is this consistent with what others have in their stock of 789s ?
Yes I know that 790 million were printed over a 5 year period and variances
occur during long print runs especially 50 years ago.
But it still interesting to me since it shows that the printer changed
something (screening) in the printing process of this particular stamp production.