Extraordinary claims require extraordinary proof. That's axiomatic. And the onus of proof is on the person making the claim.
As I understand it, there is existing documentation in the form of an invoice that four hero phasers were commissioned and built. That's inarguable. Those four hero phasers have been identified and seen on the show, and their distinguishing marks have been traced back to four known physical props (counting P1 and P2 as 'one prop'). Also inarguable.
So, all the available evidence is of four hero phasers, all of which are accounted for.
Now, the claim is being made that a fifth hero phaser (both P1 and P2) exists, additional to, and different from, those documented four. Again, as I understand it, there is no known contemporary documentation to support this fifth phaser, and this fifth phaser has never been identified visibly on screen. Nevertheless, this 'extraordinary claim' is being made. It may be so; it may not.
So it's up to those making the claim to prove that claim. In the absence of incontrovertible evidence (not imaginary shy residents of San Francisco), the onus of proof is on you.
Yet, when asked for the proof for your extraordinary claim by those who are doubtful in the face of the lack of any such evidence; when asked to prove that this fifth phaser exists, you're the ones saying 'no, you prove it doesn't'. You're the ones asking others to prove a negative.