1915 censored soldiers cover

So was the Canadian War Tax stamp used to tax mail to help pay for the war or could it be used as postage by itself? I am not at all familiar with the War Tax stamps.

Sorry for the delay responding. Yes, stampgeezer is right.
The Special War Revenue Act of 1915 imposed a 1¢ tax on all letters and postcards mailed in Canada in the hopes of raising $8 million to help pay for Canada's war effort. The one and two cent Admirals were modified with the words WAR TAX and were for this purpose. But the public and even many postmasters were confused as to their usage. The Post Office sent out numerous missives to the PMs but they were badly worded and somewhat vague. So many stamps got through strictly as postage. Early in 1916 new stamps with the 2¢ postal rate plus the additional 1¢ tax were issued to make it easier.
The cover above shows the earlier 1¢ tax plus the 2¢ drop rate. Below is a 1917 cover with the single 2 + 1¢ stamp. Also below that is a 1915 cover with a single 1¢ WAR TAX  stamp. This second cover  would in fact have been underpaid, since the tax is covered but not the postage.
I guess it should be noted that the two covers here, with the proper postage, are soldiers mail. Once these to letters arrived from over seas, from Field Post Offices, the extra postage and tax were added in Montreal and Toronto,respectively, by people who obviously knew what the rate was.

There was also confusion with a different series of overprinted War Tax. The PO took 5, 20 and 50¢ admirals and over printed the words WAR TAX and INLAND REVENUE WAR TAX in black (5¢, 20¢) or red (50¢). These were especially confusing. They were to be only used as revenue on larger items such as wine, perfume, medicine or larger documents. But again, postmasters were poorly informed and some ended up as postage.
There were also specifically designed War Tax revenue stamps, for documents, cheques etc, as well as the wine strips, which I posted on another thread.

Good question. I had posted the YMCA cover on SCF and posed the question about why there were stamps over top of the FPO and censor tape. There were a number of responses and a good discussion. One member posted a link and an excerpt from Postalhistorycorner.ca, who said this, "Postage was not required on mail from Canadian soldiers in England and France. Until July 28, 1917, however, Canadian Exchange Offices for overseas soldiers' mail affixed Canadian postage stamps on stampless mail received from soldiers overseas. The stamps were not paid by the sender or the addressee."

So still somewhat of a mystery, and will need some research in day/weeks/months to come. Will be looking up Canadian Exchange Offices and their purpose.

the postal history site is

postalhistorycorner.blogspot.ca/2011/05/george-v-admiral-period-ww-i-military.html

A couple of additions to this thread. Another military cover with wartax added after the fact, and a postcard which was returned to sender for lack of a War Tax stamp. Another 1¢ admiral was added, though not a war tax stamp.

Nice covers James, the proper use of the war tax stamps is particularly hard to find! I am sure the confusion among the postmasters in their time of use has something to do with that. And I agree, the Edward VII PSE with a George V stamp on it is pretty cool.