Besides the cosmetic differences between the different receiver types, there's also one fundamental difference between them. The type I was introduced as a forged and machined receiver. In full auto use, it was found that reinforcing the area around the rear lug was necessary. This became the type II with the distinctive scallop towards the rear. Otherwise, the type II retained all the lightening cuts of the type I receiver. Over time, Fabrique Nationale realized that producing a receiver with all the extra work milling out those lightening cuts was too expensive. They introduced the type III cast receiver as a lower cost alternative. In theory, it won't last as long as the forged type I and II's. However, I don't recall anyone ever firing enough ammo to destroy one.
With that said, the type III receivers by IMBEL of Brazil are supposed to be forged, not cast. But be aware that many domestic FAL receivers are cast (Hesse, for example). DSA receivers are forged and milled from a 19 lb billet, while Entreprise and Hesse receivers are cast. There are a few other US FAL receiver manufacturers but I'm not sure what manufacturing process they employ.
Several companies produce type I's. So far only DSA produces type II's. Entreprise produces type I's and III's (their type III is much cheaper than their type I). Williams produces an alloy type I, while FAC (Federal Arms Co.) contracts someone to produce receivers for them. Also, Olympic Arms and DPMS produce FAL receivers.