I'd have to see a photo of the valve chamber to help answer your question about a rib. The information about military engines is limited but I can say that they have similar characteristics to the 41A and 59A blocks since both have the unused boss near the oil pressure ports on the back side of the block. Not all post war 59 series blocks have the "59" on the back bell. Some Canadian engines are out there too but they are marked on the front for the most part. Many of the wartime Universal Carrier engines had the third boss opened up for it's special oil system requirements. Some truck engines had it too for bypass filtering systems that differ from the normal types.
We used to have a really good Flathead ID with photos thread on here but it has lost a lot of the photos to Photobucket. Here is a link. It still has some uploaded photos though so it's not totally lost.
https://www.fordbarn.com/forum/showt...5301&showall=1