Yes, the D-Day stipes seem to be a mistake, I can't see the RAF putting unarmed PR Spits up in the middle of the action,
unless the went in after things quietened down, to see what had happened, then th estripes make sense. The need for camouflage would be largely redundant, but they wouldn't want to be shot down by Allied fighters who had control of the air by then.
The PR Spits were indeed specially built for the job, and standard aircraft couldn't have done it without conversion. The fighter variant had specially reinforced wings to absorb the recoil from the machine guns and later cannons that were fitted, otherwise they'd have fallen apart when the guns were fired (a discovery made after test flights). The PR variants had the space freed up by the omission of gun structures for fuel tanks, and even more cameras in some cases:
Image shows an RAF Photographer fitting two F24 8" focal length cameras into a blister under a wing of a Mk Ic Photo-Reconnaissance Spitfire. To keep the current balance of the aircraft, the other wing had a 30 gallon extra fuel tank fitting, again in a blister under the wing. Approx date of his is 1939 - 1940
The area behind the pilot was also extensively modified to take the cameras fitted there, which were at least as large and heavy as those shown above. I believe this camera replaced a fuel tank, and was positioned there because of the effect it had on the centre of gravity and performance, and this was ultimately why the armament was removed and replaced by fuel tanks in the wings.