Peter OK here is the rest of the cover but it will not add any more information. It is one of many although with an unusual hand stamp most are much smaller. These covers are not uncommon. The POSTAGE PAID / PORT PAYE system was used in Canada from the mid to late 70 for at least 10 years.

The only common theme is that they are all addressed abroad and underpaid. With or without return addresses. You can see many others on
ebay. Some covers have these type of hand stamps and also a postage due marking. It is confusing.
For a system that was quite widely used over a reasonable long period information about is is proving hard to find.
AQ