I had this car as a puzzle myself. You can see the article from Motor-Kritik and the explanation.
Now if you look on the web, you can find the P3 registry quite easily. In there they offer detailed history of every P3 except this one: Alfa Romeo Tipo B (P3) s/n 50008
So, my guess is that this is the ex-Caracciola P3 s/n 50008
Just an educated guess