Additional comments from my Neos notes...
I think some of the questions to ask are "does the Beretta 'kit' meet the criteria specified in the ATF letter?" and "what is ATF definition of a 'single source'?"
If accurate that the NEOS kit contains a shoulder stock only and a +16" barrel only then it does not appear to meet the ATF definition of a kit. Then move to the "single source" issue - what is that specifically? The same store, the same distibutor or just the same manufacturer? Is a "kit from the same source" a single purchase of all items at once or can it be from the same manufacturer over a period of time? A strictest interpretation of the kit and single source requirement could be buying all the parts specified by ATF at the same time as a complete kit. The loosest interpretation might be buying all the parts from the same "source" (meaning manufacturer) over a period of time. I don't know the answer.
We then have to deal with the ATF requirement of the receiver not being previously assembled as a handgun or a long gun. According to the ATF letter if one purchases a NEOS frame but does not assemble it as a handgun first - they're fine. If one purchases a NEOS already assembled as a handgun - not fine.
Regarding the "a company would be too careful and not do this" I'd like to think so too. Makes sense to me they would know what to do and not do, big $$$ and PO'd customers are on the line. It would be interesting to get their take on the ATF letter. One could interpret Beretta's responses to the questions already posted to mean "if the frame was purchased as a handgun with the barrel not attached and if one purchased our "kit" at the same time you can switch back & forth between handgun and long gun". I don't think they wrote anything contrary to that strict interpretation, but that's not exactly answering the questions in a most helpful manner.
I think the scenario gets complicated & requires precisely worded questions & follow up for accurate and complete answers. I think an "all in one" complete single source kit complies with the criteria in the ATF letter allowing back & forth switching. As one strays from that all in one complete single source kit - I think it becomes more difficult to give a certain answer. I think separately buying a NEOS handgun a few years back, then separately buying and attaching the NEOS carbine kit, does not appear to meet the criteria specified in the ATF letter. That's just my layman's read of the thing.