Deep inset skylights,
with rubber seals set below the roof line (pre 1953).
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