We know after Canada became a country in 1867 and Newfoundland became part of Canada in 1949. The Government allowed the use of Newfoundland stamps on covers mailed in Canada. (as far as I know the law is still in existence today)
Below is the standard rate before Oct.1968.
Below is the cover in question that I was interested in.
Now if you notice on the close up, there are two Scott 337
stamps originally placed on the envelope...The he/she placed a Scott 392 FIVE cent stamp over top..That should have been ok to go, BUT placed THREE Scott 233
Newfoundland stamps around the others..Now this is overkill...Should have stopped when the FIVE cent was placed or THREE more Scott 337
...Just a strange cover/envelope.