In the past, I've always gone the more traditional route of filing a form 1 and "building" my SBR.

What is the process if you need a gunsmith to do the work required to transform a non-NFA pistol into an SBR (welding, fitting, etc.)?  Who files for the stamp and are additional transfers required? What should be engraved on the item?

Whomever is the "maker" is the one whose information has to be engraved. If the FFL is the maker or manufacturer, then it'd have to transfer to you on a tax-paid Form 4 after the Title 1 gun/parts are made into a Title 2 SBR. A manufacturer FFL is required. If the FFL is a manufacturer but not a SOT (not common, but it can happen), then the manufacturer would have to do it on a tax-paid Form 1, before initiating the tax-paid Form 4 to you.

If you make the SBR yourself, then you're the maker, and your information is what gets engraved. You do not actually have to do all the work yourself, but you do have to get it past the "making" point. For instance, you could take an existing rifle, file F1 and then on approval cut the barrel with a hacksaw to below 16", then send it to an FFL to be properly cut and crowned and maybe have the gas system tuned or adjusted. That is considered gunsmithing work and does not require a tax-paid transfer either way, though some FFLs will want to use tax-free Forms 5.

Further, if you are able to assemble all the parts together to complete the firearm, except for assembling it, then that is considered to be the firearm "in disassembled condition". It's where the legal definition of "constructive possession" originates. That's not entirely a bad thing, since if you file your F1 for something like an HK SBR which needs the trunion fitted and welded, you can simply wait for the F1 stamp to come back, then get your short barrel and engraved receiver together with the complete parts kit, and that assemblage is legally considered to be the SBR, which can then be sent off to a gunsmith to be assembled and refinished and whatnot, again with no tax-paid forms needing to be involved - just F5s if that's how the assembler wants to do it.

@Circuits

Had a question about what you wrote. So your saying a non SOT could manufacture an approved F1 for another individual? So if that were true the same could be said of a suppressor right? just doesn't add up to me.... I looked into getting an 06/02 and they weren't too bad but the part that kills you is ITAR. and to my knowledge even if you make and sell 1 thing your "in the business" and have to pay ITAR. I mean you could make title 2 items till your blue in the face for testing and R&D and even give samples to local LE from what I gather but the second an item is to be transferred and money is involved ITAR will be up your arse.

If you know something I don't know I'd love to hear.

This would fall under the BATFE MFG FAQ of 08/15/2008:

6. A company acquires one receiver, assembles one firearm, and sells the firearm.

The company is not manufacturing firearms as a regular course of trade or business and is not engaged in the business of manufacturing firearms. This company does not need to be licensed as a manufacturer.

The keyword is "one".

No... the non-SOT could file the F1 for themselves to manufacture the item, then use an F4 to transfer the completed item to you, the buyer. They cannot take YOUR approved F1 and do the making for you. See also RenegadeX's reference to limited manufacturing - the non-SOT couldn't do that as a regular course of business. I'm kind of sorry I even mentioned it, because it's a corner case and not typical or usual.

The situation in which you "complete" the making of your F1 item into a legal NFA firearm first, then send it off to an FFL to work on it is distinct from making/manufacturing for someone else.

Suppressors are a weird case, because ATF treats their parts (baffles/tubes) differently from other NFA firearms, and you were asking about SBRs specifically in your original post.