Pure speculation...but I think if it was cast with that it was using casting cores that Ford introduced for WWII full flow applications. As far as I know, that little circle appeared while the 1942 engines were still in use, presumably during the war, and remained on there until 1953. Canad continued to offer the WWII full flow setup as an option after the war. I don't think I've ever seen it on an engine earlier than the '41-2 types, and many of those lack it. My guess is that the ones without are 1942 civilian and early war ones... but still a guess. I also think of the manifold pad as a '41 and later thing...but here I cannot say about USA 1937's as I have no experience with those. I do suspect it is something that was picked up after '41 with later casting stuff... I don't know my British 77-78 engines at all well, either...I cannot say whether those picked up the pad or the boss. Most wartime Bren carriers were, I think, Canadian with some USA engines fed into that stream as Ford Canada was much bigger than Ford England. Carriers had full flow coolers back there...but I am not at all sure about early war ones or British built ones with 221. We need some input from military vehicle nuts! And of course wartime stuff is muddled by numerous supply chains on the allied side feeding vehicles with interchangeable motors, and on the German side by vast exchanges of B and V8 vehicles in Russia and Africa...both sides were very happy to repaint any B or V8 Fords they could get in the huge sweeps of 1939-42.