Just to be clear on something....this house 100, it is not a brokerage firm like Quest, BMO, RBC, Jitney or any other.  This house 100 is a function of the TSX to satisfy delivery failures by others.

Which brokerage did the short seller or short sellers who've failed to deliver shares use?  I don't know and I don't believe there is any way to know.  

But the possiblity of a margin call increases with the PPS so long as the short seller hasn't covered the short position.