1968 Barracuda Notch - Plano, TX
I have heard the term before, and that is also what Sam called it, may not be common terminology. The stock frame has a few holes in it and was also a bit beat up. It looked beat in a little, kind of dished, and looked like some one had pulled it in a few spots with a hook, possibly with a tow truck. Sam welded a piece of flat plate over it to add strength, make it easier to weld the torque boxes to, and for cosmetics. Without the plate there it would have been a pain to weld the torque boxes or he would have had to beat them down to follow the contour of the damaged frame which did not make sense.
Here is the frame after media blast, I have arrows drawn so you can see the holes:
View attachment Frame Before.jpg
Here is the front with a plate welded in place and the torque box before primer:
View attachment Front.jpg
And after:
View attachment Front After.jpg
Here is the rear with the plate and torque box welded:
View attachment Rear.jpg
And after:
View attachment Rear After.jpg