You as the Purchaser are present when the form is filled out. You’re there when it’s completed and you don’t SIGN it unless it’s complete and accurate.
With a lower receiver, as stated previously, you have the FFL mark it off as “pistol” or “other”.
There are some lowers that are manufactured as “rifle”, but almost all today are gender neutral until the form is filled out. Yeah, my tribute to diversity this month.
If the business where you are purchasing over the counter or conducting the transfer (the FFL) says he will not mark it as pistol or other, then you do not complete the sale. Don’t say “I don’t know” when it’s time to fill out the form.