Details & Changes.
 Deep skylight.
Old deep skylight.
Later skylight.
Deep inset skylights, with rubber seals set below the roof line (pre 1953).
later version with rubber seals flush to roof line, less inset (post 1953).
It may be a little hard to see in the pictures, but the profile is something like in this drawing.
Searching through the picture register, I have found that the change is between chassis number 20-044995 and 20-049493.
If anyone knows the exact date of the change, please let me know mail