Well, this is bullshit

Today, we became aware that on or about January 9, 2020, a letter sent out by the Pennsylvania State Police (“PSP”) to Federal Firearms Licensees (“FFLs”) that pursuant to what it refers to as “the Pennsylvania Attorney General’s binding opinion”, the PSP has promulgated an “Application for Purchase (Partially-Manufactured Frame or Receiver for a Pistol or Rifle)” form, SP 4-121, which must be completed in addition to “a PICS background check and unique approval number [being] completed and received in order to facilitate a lawful sale/transfer of a PMFR [(“Partially Manufactured Frame or Receiver”)] under Pennsylvania law.”
...
 But, what are PMFRs? Neither the letter nor the form define them in any way and all licensees are left to guess whether a non-object firearm constitutes a firearm under this undefined term. Thus, the PSP wants dealers to “guess” at what constitute PMFRs and if you get it wrong, don’t worry, you’re only looking at felony offenses…
Politicians who say "We won't tell you exactly what this means, but you have to obey it, and if you guess wrong you go to jail" need to become lamppost decorations.