With all due respect @TacticalMedic , you are offering incorrect direction. Although 922(r) is rarely, if ever, enforced as a primary offense, where you bought something in terms of compliance with 922(r) is irrelevant.
18 U.S.C. § 922(r)
It shall be unlawful for any person to assemble from imported parts any semiautomatic rifle or any shotgun which is identical to any rifle or shotgun prohibited from importation under section 925(d)(3) of this chapter as not being particularly suitable for or readily adaptable to sporting purposes except that this subsection shall not apply to -
(1) the assembly of any such rifle or shotgun for sale or distribution by a licensed manufacturer to the United States or any department or agency thereof or to any State or any department, agency, or political subdivision thereof; or
(2) the assembly of any such rifle or shotgun for the purposes of testing or experimentation authorized by the Attorney General.
If you turn an SP5K into an SBR, you are assembling a weapon (making) that is otherwise prohibited from import and are required to insure that the weapon does not have more than 10 imported parts. Whether you do so or not is up to you. But where you bought the base gun has nothing to do with it.