I believe the P did stand for 'prototype' generally. Unfamiliar German types (for which pilot's notes were surely available in post war testing) were similarly and distinctively marked, typically with yellow undersides.
Pilot's note's, the vast majority being relevant, were certainly available for these two 'P' marked aircraft.
I suppose the pilot could ignore the section on the undercarriage of the Spitfire float plane. The system was removed to save weight, so he'd never have found the controls
I don't believe that by the time of that photo (1945) there were any LW units operating the 210 - just the 410. If there were any 210s I think only Hungary were flying them.
Caption translation google: A new aircraft of the German Air Force is created.
In the battles of late has been on all fronts, a new multi-purpose aircraft, the 'Messerschmitt ME 210', proved to be excellent. With this machine, which can be used as a heavy fighter, reconnaissance or destroyer, the German Air Force, a new powerful weapon was created, deserve special mention, nor its great speed and powerful armament.
Partial views in one of the workshops in which the fuselage assembly which takes place in hergestellen Grossserienbau 'ME 210'.