Several sources, most of them before my time.
I've been told some are better than others!

The last megablock foundry I know of was motor castings, the last machining facility I know of was cummins recon.
I know the least about it because it was on it's way out when I got hired.

When the one folks call a world block came online in '08, Dalton Foundry (casting mark on the side
backwards D with an F) cast them and Royal Oak boring did the machining.
The only trace left of the machine shop is in a supplier code/long since forgotten.

Just look for the two freeze plugs in front.

I recently noticed...on the later "world" type, you can tell wedge from hemi by looking at the front only, due to one machined hole the hemi has (that the wedge doesn't). I'm not sure I should post anything about that, as it's pretty high level geek material. Nah, just look at the freeze plugs.