I've been working on several heads lately mostly F series tractors (regular, F20, and 1020) this is the first one i recall to have a casting date code on the head.
it's an F20 head, painted red and looking very original, and has a casting code of 5 / 26 / K. 1941 or 1942 depending on if O was used. But 1939 was the last of the F20's, and that was probably just leftover production castings.
i don't have the rest of the tractor, this head and manifold was in the barn out at Zagray. We have a couple of guys fixing everything in sight that has a hand crank, so i've been stripping the heads and sending them off to my machine shop bud for surfacing and inserts / knurlizing the guides, etc.
i imagine this is a replacement head, made during the war years.