The law has always read that a virgin receiver could be used to build either a pistol or a rifle.
The 4473 only had a spot for 'long gun' or 'hand gun', which technically, a receiver was neither. Folks who planned on building stuff would tell their FFL holder to mark their reciever purchases 'long gun' or 'hand gun', depending on what they were gonna make.
But legally, once you got your "long gun" (so marked on the 4473) home and seen someones whiz-bang AR pistol on some gun forum and changed your mind, you could legally build a pistol on your 4473 marked 'long gun' receiver, since technically it was still a virgin receiver.
The new "receiver" check box is just clearing up possible confusion.