I've seen .303 Proof rounds marked with "Q" headstamps, base stripes, case stripes, bullet rings, copper-washed cases, and some early MkI and MkII rounds with "PROOF" in the headstamp, but never the Canadian variation originally posted. If I had one to put on my table at a cartridge show, I would start out at USD75.00, or even a bit higher.
I'm not a serious .303 collector, so they may be out there, but I've never seen one like that.