My copy of that picture credits Maxwell July 2nd, 1941. This is the latest picture I have of it in red. It was also published as a Vanishing Vistas Post card. My pictures leave a pretty good gap in painting OY.
The first picture I have of it in Gray is grungy looking in February 1946. Pictures of the rotaries during the war are pretty scarce. Here is another probably taken the same day in B&W.
The first OM in Gray I have is January 1946. Maybe Jerry has a better date nailed down for either of them.