History
This was even before my time!

Look at the dog on the left.  She is doing the box the way it was intended in those days.

The poodle on the right was more agressive.  She attacked the tuna can.  This left a lot of room for potential hazards.  The slot in the box was one, and being wacked in the face by the tuna can was the other.
(white shoes and black pants was another hazard!)
I am missing a few in between shots here.

This was Marti's Monsterbox.  Only used once in 1986.  Debbie's BC, Bingo, had been banking off Marti's legs when they just used the "Sureshot" box.  Marti added the back plate to save his legs.  Of course there was a great outcry from the masses.

But, as a result of this, his neat innovation led to angling of the box itself.