Donald,
What you're seeing is probably the burelage underprinting that was used on some of the Wood Pigeon Airmails.
Here's a side-by-side of stamps without and with burelage.

There were four different sets of the Wood Pigeon Airmails.
Scott C3-C11 (Jul 1922) -- C8-C11 with burelage
Scott C12-C14 (Feb/Apr 1923) -- all with burelage
Scott C15-C19 (May-Jun 1923) -- no burelage
Scott C25-C26 (Jan 1924) -- no burelage