I'm not sure that the Petard is actually a mortar. I believe that it is a hollow charge weapon that is placed against a target by a boomb arangement on the tank.
The Petard is actually an ancient weapon used to blow holes in castle walls or gates. The expression "Hoisted by one's own Petard" comes from the fact that sometimes the bomb exploded prematurely and the person igniting the bomb was blown up in the air by the explosion.
The only picture I have seen of a WW II Petard is in one of the War Amps videos. I believe Cliff Chadderton gives a brief description of it in his new book "EXCUSE US HERR SHICKLGRUBER". The book is available from the War Amps for $10.00 well worth it.