That is interesting about the no brace light casting because you are right in this being a pre-73 detail. All I can say on this is that there must have been castings and components deliberately placed in containers at the begining of full production of each model and placed in the stores for such time prototypes are built up for possible future models. These parts are then sourced from stores and used by the tool makers Fred and Ken for mucking around with, hence the use of the same Y-2 Vauxhall screen on the Y-6 Rolls-Royce Fire Engine (no un-braced casting found so far on a Fire Engine).