There are three types.
Type 1 and 2 both have machining cuts. The profile of the cuts are different. Type 1 receivers were found to crack under HEAVY full auto use in the FALO squad support weapon. The lightening cuts on the aft section of the receiver were changed to strengthen this area. The Type 2 receiver has the stronger lightening cuts.
Type 3 receivers eliminated the lightening cuts. It greatly reduced the cost of production of the receiver, as there was less machining. It also didn't sacrifice any strength.
In practical terms, there isn't really going to be much difference in what you notice as a user. The machine cuts make the receiver lighter, though not by much, and not enough to really notice on a 9+lb rifle. The Type 1 would technically be the weakest, but failures were limited to the FALO test pieces that fired a tremendous amount of rounds through them on Full Auto. I don't think there were ever any failures associated with the cuts on standard rifles, even ones that were tested by the 90+ coutries that used the FAL. So unless you're building a FA heavy barrel FALO, I would just go with whatever profile you like the appearance of.
As far as strength goes, it's more important to chose a good quality manufactured receiver than what the milled cuts are.
If you're building it yourself, get the best receiver you can. The build will go much smoother if the parts actually fit in the receiver. Quality varies greatly. It's worth paying the extra $50 or so to end up with the right part.
Go to
www.fnfal.com for more info
Ross