And it was right under my nose. I rode my rebuilt Sentinel to my LBS today, and the owner said, "Oh, yeah -- we used to sell those; we got them from a company called Wilco in San Jose. They sold for $89 in 1973 (the year mine was built), but they were only imported for a couple years and not in any quantity. He didn't know which Japanese manufacturer built them though.
Here's mine, fresh paint and wheels too good for it.