There used to be one on YVR-YYC as well, C3OW if I recall correctly, not a bad deal, in my opinion. However, it seems to have been replaced with MBOOKC - still booked in C class, but at the price of an M fare.
It's funny that AC has loaded another M fare on YVR-YYC, MSTRB, which is the same price as MBOOKC ($221Cdn each way + taxes + surcharges), but MSTRB is only good on ZIP flights.
Hmm... J on mainline vs. economy on ZIP... tough choice.