I don't think anyone would be able to give you a definitive answer on that, at this distance in time.
But it's my opinion that they would have been fitted to the end of the model run, as they were still available as a spare part for years afterwards.
A very slow-moving one, as the cover didn't perform any useful function really, and by the 1970's owners didn't care much about the cosmetics, on these older mowers. We still used to get a few Model 5's in for service in those days, though. Those things were really hard to kill!
