Hope you don't mind all of my various postings in here but I do enjoy going through the threads and helpuing wherever I can.
One of the scans in this thread illustrated a "Blackout" cancellation. One can make a specialised collection out of this type of cancel alone.
There were East and West coast cancellations plus those from the major cities within Canada as a whole. They were created so that should mail fall into enemy hands during the WW2 conflict, that they would not be able to tell where the lettor / card origin was. The location portion of the cancellation was always omitted. Some CDS types were produced and some were slogans, such as the one listed here.
This was defeated many times because in a lot of cases people would not adhere to the rules at the time and would either use their letterhead (commercial envelope) or write their return address on the cover anyway. This was done usually out of ignorance of the law, forgetfullness and, yes, on purpose by the enemy so that these cancels could be traced.
There is not enough space in here to elaborate on these but there are many articles in the different journals of the different societies that explain these cancellations fully.
If one can get their hands on some of the better Canadian military postal history books, you will understand what I am getting at.