I know what the absolute right answer is.  Start with a new receiver.  Barring that, this is my question.  

I did some gun swapping locally about a year ago.  One of the items was a used AR receiver.  The receiver was traded stripped (except for bolt catch).   It is a spikes receiver that is generally sold stripped.  I honestly do not know the history of this receiver.  It does show wear marks so it looks like it was fully assembled and used.  

I acquired this receiver a couple of years ago.  

I'd like to make this into a pistol with a brace.

Best you can do is call Spikes and verify they did not sell it as a rifle originally. Past that neither you nor anyone else can show it was ever a rifle.

It's not perfect but as long as spikes says it was not built as a rifle when it left their possession I'd be comfortable building it as a pistol.

That's a great idea.  I'll do that.

Whenever my friends aren't looking, I always slip rifle stocks on to their lowers and then quickly remove them before anyone notices or detects my devious subterfuge.

Doubt it makes a difference either way!!!

:)

I know it.   Doesn't make any difference.   I try to stay on correct side of the law.  

I donspeed sometimes though.

But, yes OP, you're right to make sure the firearm was not built that way by a manufacturer. That is a sure indication the ATF will know the same and that you don't want to mess with.

I'll call Spikes tomorrow.   I didn't have the serial number on me.