It is not a door. On the Cessna 210 Centurion models the left side of the fuselage has an openable (as photographed) window standard, optional for the right side at opposite position. This is unlike the Beech 36, which both side windows over the wing are NOT to be opened in flight, and are so labeled for a passenger to read/heed.
The Centurion name was introduced on the 210D model, the photo is of a 210E Centurion.
Perhaps it was a hot July day at OSH. Been there, done that.