Beverly Hills Historic Preservation credits the photo
e_r posted to CC Pierce and dates it to 1920 (which still seems a bit late).
The "Beverly Hills" sign was already gone by the time of this 1929 photo:
Undated photo of the lilly pond (no sign):
The sign, the lilly pond and the house (pre 1929)
The old sign spelt out "Beverly Hills" in lights. The new sign, alas, does not:
Panorama from Beverly Drive and Santa Monica Blvd, 1932:
1309 Park Way for sale, ca 1920:
cc pierce
All pix Beverly Hills Historic Preservation -fb page