The diamond was just a foundry mark on the model A engines that were subcontracted after Ford quit casting them. These were made up till 1941 or so. Since there were several companies that used variations of the diamond shape as a foundry mark, no one has determined exactly what company cast them.
There were only a few companies that cast the V8 engines out side of the Canadian, British, French, and German Ford manufacturing facilities due largely to the complexity of the casting process for these engines. I've never heard of any company being subcontracted to cast them for Ford Motor Company like the model T, model A, and model B were. There are a lot of unknown casting markings on the V8 engines over the many years of production.
SUMB in France cast the hybrid V8 engine on there own to get the Military contract to produce the french military trucks. The old french Simca 60 HP engine design ended up with a South American Chrysler related company Simca Do Brasil to produce an overhead valve version of that design that is similar to the Ardun conversions.