Just a follow up.
The Library and Archives Canada responded to my inquiry:
"I was able to photograph the front and back of our 100 stamp plate proof. Please find the images attached. I'm afraid that the inscriptions appear to be modern ones made by curators and/or archivists for cataloguing purposes, rather than the printers or the Post Office Department.
Unfortunately, our luck was about the same with my colleague who works with private collectors; he wasn't able to provide any further information.
In spite of this news, I wish you all the best in your research! Thank you again for contacting Library and Archives Canada with your question."
The work they did to research my request was impressive! It's not something one would expect from a US government office.
My question remains a mystery although it looks more likely to be a collector's notation.
Dan
