Sooooo.... what does a 50KW low pass filter look like?
Every BC transmitter built in the last 50 years has very efficient harmonic suppression built in.
Two reasons: Regulations spelling out just what levels are allowed, and that energy going out the line costs $ to make.
A link to a treatise on the subject:
http://www.hatdaw.com/papers/review_of_harmonic_suppression_practices.PDF Canada pretty much follows the US in technical broadcast rules.
A BC antenna matching unit is typically a "tee" net, providing a match from 50Z to whatever the tower impedance is at the op freq. That further 'losses' the harmonics, but at that point they are many dB down already.
The station in Toronto must have a really dicey antenna system to cause a 3rd harmonic of that magnitude, maybe something corroded causing re-radiation.
73DG