Any new, stripped AR lower may be made into a pistol. There is no ATF requirement that a lower intended to be made into a pistol be marked "pistol".
Last October ATF changed Form 4473 to include the option of "Other" when designating what type of firearm was being transferred. The previous form allowed for "Handgun", "Long Gun" or "Both". ("Other" replaced "Both")
A lower receiver that has not had a stock attached is not yet a long gun. It is also not pistol. It is an "Other" until YOU decide what to build.
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$20 transfers ($10 for CHL, active military,police,fire or schoolteachers)