Here's what the July 7, 2009 open letter states:
1) An FFL may not sell a frame or receiver to anyone under 21 years of age [Title 18, U.S.C., section 922 (b)(1)]. Comment: This is because the receiver could, potentially, be made into a pistol.

2) An FFL may not transfer a frame or receiver to an unlicensed person from another State. [Title 18, U.S.C. section 922(b)(3)]. Comment: Again, this tracks the rules regarding transfers of pistols, because some frames/receivers can be built into pistols.

Full text of letter is here.