Figured I'd add these newly acquired soldiers covers to this thread, since they show two of the censor marks shown by Rod above.
Toronto postmark March 17 1915 predates The Special War Revenue Act (War Tax) by 10 days (came into effect March 27)
Military Field Postmark underneath has the date obliterated by the Toronto mark, but the code 2X indicates it was sent from the 1st Division 2nd Brigade.
Red square censor mark (CM2). The Toronto mark has a nice CNE (Canadian National Exhibition event cancel.
A lot going on here! Cobalt Ont. receiver cancel on back
Also on the back you can make out a 'negative' impression of some handwriting. That is a name and address from another letter, probably transferred from another envelope while they were stacked. It reads Geoffry W. Dixon Box 271 Cobalt Ont.

This second cover dated November 2 1916 at Army P.O. 3 has a six sided censor mark (CM4). The MR4 Die II war tax stamp was applied at the Ottawa Exchange Office on November 13
Also back canceled in Cobalt. This one also has some interesting marks ENGINEERS & MAPS COBALT ONTARIO.
