Your standpoint, and a buyer's standpoint, are two completely different things, brother. Completely. You're saying something has to be transfered as a rifle or pistol based on what gets stuck in it's butt. Not the case. Maybe for you, depending on what you stick in it's butt - but the buyer doesn't need anything stuck in the butt. They just need a lower, so they can move on. That transfers as "Other," and it definitely isn't a pistol or rifle on a 4473. It's just a lower, and it's "neither" at that point.
Don't confuse people with the steps that you need to make, as a retailer - when most of the people here just need a stripped lower. If you screw a pistol buffer into a lower in the shop, and someone comes in and says "I just want that sweet billet lower!" ...does that mean that since you inserted a pistol receiver extension into that lower, that you need to transfer it as a pistol? Nope. It's never been barreled. It's never been "built," until it's been barreled. That stripped lower must transfer as "Other" on the 4473.
Let's say you build a complete rifle, with a MagPul PRS on it, rifle receiver extension, 18" barrel. You have that built rifle on the wall in your shop, on a Mega ambi billet lower. Mega transferred that to you as a stripped lower, and records prove it. Dude walks in off the street and says, "I need a Mega ambi billet stripped lower!" You tell him, "Man, I'm FRESH OUT! Sold the last one ten minutes ago. Only one I have is on that rifle on the wall. Gonna be 10 weeks before I get another shipment." Dude says, "I don't need a whole rifle, just THAT lower, man! I'll pay you $200 over the MSRP of THAT LOWER, just so I can complete my build!"
What do you do, FFL?
Shipped to you as a stripped lower, on transfer documents. You assembled it and it hung on your wall, as a display - but it's never transferred again. Do you rip it down to the stripped lower and transfer it as "Other?" Do you make the guy buy it with the rifle receiver extension in it, and transfer it as "Rifle?..."
I already know the answer to this one, Shib. It IS a trick question.