[Joe K]

May be "chappets" (I think I have this term correctly spelled) which are steel or cast iron "spacers" which hold the core in place during pouring of cast iron. VERY commonly used on intricate or involved cores to prevent shifting.

Normally these are sort of "absorbed" by the molten iron and you don't really see them on the rough cast surface.

I would certainly pressure test the block you have simply to assure a good starting point for your further work. This test will reveal any flaws. Don't go too high on pressure lest you upset an existing "rust joint" which will henceforth be a leak - at least until the rust joint re-seals (that is if it re-seals.)

Joe K