I am purchasing a 1923 building at Clay and Larkin in SF, CA, with
what the City contends is an illegal roof top unit on it (about 300
square feet). I do not want to have to tear down the unit.  To help
save the unit, I need to show that the unit was on the building when
it was built, and has always been there.

I have gotten 1946, 1956, 1968, 1973 and 1974 aerial photographs and
scanned them from the USGS (Menlo Park, CA) and a 1937 photograph from
the SF Public Library (History Room), and I have also gotten a 1993
and 2004 photograph on line from www.terraserver.microsoft.com.

Of all these photographs, only the 1973 and 2004 photographs are clear
enough to show if anything is on that roof.  In both cases, the
photographs show the roof top unit.

Do you know of other sources or ways in which I can get clear aerial
photographs that would show this unit on the roof if it was there --
dating back to 1923 and onward (or for as many years as possible,
beginning as early as possible)?

Hmmm?  At least back as far as 1973, over 30 years.
Has the building code been changed since then so that the roof unit
might have once been legal?
Even if it wasn't, why hasn't the city pressed for removal earlier?
Has the property changed hands in that interim, so that if that was
the trigger to let the city insist on removal, there is an argument
that a precedent has been set?
Could it be argued that the burden of proof lies with the city if it
has allowed the roof unit to remain so long?

An expert familiar with construction should be able to recognize if
the unit was part of the original construction or later added, and in
that case, may be able to suggest when, maybe supporting an argument
that it was added at a time when the code allowed it.

In dealing with the bureaucrats, I wouldn't bother them until I were
completely sure of my line of defense.  The more often they get a
chance to say or write that you are wrong, the more difficult it will
be for them to reverse their decision  (just human nature, accentuated
by the view of a file of refusal letters followed by one admitting
that it was OK).

Records on the building?  Construction, previous sales showing areas,
nos. of floors, tax records,  ....?

What do architectural historians say about it?

Any other sources of photos along either of the streets?

Am I just throwing out questions, or could one of the other be of help?
I hope so, especially on the principle that the city may not be able
just to take a stand with the simplistic: "Prove that it was always
there."

Good luck, Myoarin