Expansion gasket for replaceable firebox lining

There is an expansion gasket between the 2 bricks of the side walls that create the firebox on courses 7-10 “which serves as a replaceable firebox lining”. My question is why the gasket doesn’t go around the back of the firebox also, if the interior firebox bricks need replaced?

Yes, we usually put 1/8” or 1/4” ceramic paper between all three walls. I’m sorry if that wasn’t better drawn.