The Indianapolis Foundry only cast gray iron 8 cylinder engine blocks (and disc brake rotors for a short time in 72) in late 60s and 70s. Later Indy cast the iron 4 cylinder blocks, some of the late production Jeep 4.0 liter in-line six and V-6 iron blocks til plant closing in about 2005/6.
Per a Chrysler casting process booklet I received (published in 1974), there were four corporate grey iron Foundries at that time including Indy. Per that document: the Huber Foundry cast iron 6 & 8 Cyl cylinder heads, 6 cylinder blocks & ductile iron crankshafts; the Fostoria Foundry cast Intake Manifolds, Exhaust Manifolds and manual transmission cases used at New Process Gear; last, the Winfield Foundry cast gray iron camshafts and timing gear blanks.
I looked for that casting mark but there were very few product pics in the booklet so couldn’t tell if that was Fostoria’s mark or not.
Also I’m not sure too if other foundries farmed out work to other foundries if they couldn’t meet production requirements but I do know the Indianapolis Foundry had some blocks cast at the International Harvester foundry (later Navistar)also located in Indy. IH poured some 318 blocks during my time at the foundry & my brother had a 1964 Plymouth with a 383 block that had a rivited tag with IH on it when we took it out for rebuild. Tom