Yes it was a device to hold the reversing catch "on" while reversing or to be more precise let it off at the flick of a switch after reversing was completed
It was easy to lose one's cool when revering and the mechanical reverse catch released itself every time you needed to inch forward as when you then tried to continue to reverse the outfit would go up in the middle and stay stopped
I can remember on a busy overshot crossroads nipping in and out of the car to reset the non Marwood equipped reverse catch
The air was blue !!