Well ... the pic as a pistol probably can't be used to definitively prove that you first configured it as a pistol, but the fact that it was a pistol while the receiver was still 100% pristine certainly supports the argument that you are making. Is it a guarantee, nah, but I think that the burden of proof on this one will fall on the prosecution ... if for no other reason than it is almost impossible to prove otherwise ... EVEN if you did get it originally registered as a pistol ... you STILL could have gotten home, gotten excited, and gotten it configured as a rifle first!
I'm glad that someone FINALLY posted a place to look for the definitive (and hopefully documented) answer to this question ... I am tired of people posting that something is illegal without citing a reference.