I'm afraid you put me on the limb there. I don't think it is the prototype. As far as I have been able to find out the prototype's registration was NC13301 and has two-bladed propellers. And this one is clearly NC13316. And United Airlines it is, cost them $50,000!
So, pretty close.
Apart from this detail you are of course absolutely right. Well done.
So over to you P51