Yes it might be 403 Clint, but then it is a bit strange no serial number is present on the aircraft.
This was also before any censorship had been invented by the militaries, so I think we can discard that as well.
I browsed around a bit before this photo popped up and there was at least one other photo where the same photographer had used three digits for some reason, again on a 'blank' aircraft (think it was in the 300 range). Of course I am unable to locate it again....
Cheers
Stig