the identity of the mysterious building on spring street has been made, courtesy of an architectural guidebook to los angeles by david gebhard and robert winter. it’s an annex to the i.n. van nuys building. it was built in 1929-30. the book claims that the north parking entrance “retains its original classical metal gates,” but unless it is behind the not-original not-classical gate you see in the picture, that may no longer be true.

if you look at the picture, the ledge near the top of the annex does match up with the ledge around the van nuys building (on the right). i don’t see many architectural similarities between the buildings other than that.