The gap is aesthetic. It has to do with how Coonan machines the receivers; apparently their method is easier. I didn't even notice on mine until it was pointed out. The Coonans are good (they are cast), but they do lack sand cuts and a couple of minor lightening cuts. On the plus side, they have been open to custom markings on the left side and will do custom serial numbers.
The "G1" receivers have custom markings. I assume that's part of why they cost more.
If you can find a NOS Type 1 or Type 1.5 forged DSA, that should also be fine (and a few ounces lighter than the Type 2 in the case of the Type 1; marginal difference for the Type 1.5). People do have forged receivers for sale by themselves from time to time.
IMBEL and FMAP receivers are also sold on their own sometimes, but they carry a premium, coming from licenced FAL manufacturers. I've only ever heard of Type 3s being imported, though.
For a current-production Type 3, I'd go with Coonan. If I wanted custom serial numbers, I'd probably go with Coonan (DSA generally won't do them). If I wanted more accurately cut Type 1s, Type 1.5s, or Type 2s, I'd go with DSA, cast or forged. If I wanted forged, I'd either get a NOS DSA or wait to see if DSA goes through with a short run of forged they might do in the near future.