Well, it sure looks like it could be a crack. I would make a WAG that the engine froze and cracked at that point. I had one engine that had frozen and the crack was down in the gallery where the distributor shaft headed down to the oil pump. There was no good way to get to that one to fix it and that block was scrap - back in 1966.

BUT - with the brass plate and all of those screws, it may well have made a good seal - a really great farm fix! If that is the only issue, I think the patch is a better solution that trying to weld up the crack and putting heat stresses into the area.

Let us know what you find ... we're curious too.