I've seen the same thing in a couple of other parts of the Sunset, too--25th & Lawton is example. Most of the houses on that 1500 block of 25th are typical 1920s/30s construction, but the last 4 on the east side of the street are 1950s era, including double garages that are side-by-side. I believe those lots were still empty until the 1950s. However, the north side of Santiago between 17th & 18th has a different history. That spot has a trio of 1950s houses facing Santiago, but they replaced some old abandoned houses that were torn down circa 1960. At the Santiago location, I remember sitting in the car with my dad one afternoon and watching the demo in progress.