I assume by "heavyweights" you mean OB, OP. most flangers and the rotaries. The NG Pictorial VII is a great resource for all things MOW, but does not indicate when equipment was painted gray, though it does generally indicate those pieces that were not painted gray and stayed red/brown until retired. An advanced search of the forum will unearth dates ranging from late thirties to early forties for the switch to gray. So... search away. I'm sure there are other resources out there that will be more specific as to the dates gray became the standard MOW color.
Mike