He said in a few interviews that he won the tournament that turned into the US Open before it was called the US Open, so technically he won it more than the official stats for it.
I checked Charlie Ursitti's info. It shows an event called the "1st Annual U.S. Open 9 Ball Championship," played at the Jack & Jill Cue Club in Arlington, VA in January, 1969. Then it shows the "3rd Annual U.S. Open 9 Ball Tournament" at the same room one year later. Then it shows no more of those events.
Those 1969 and 1970 events would have been before Sigel's time (ages 15 and 16), but if the event continued from 1971 to 1975, it's possible that Sigel played in some of them, with the "U.S. Open" name attached to them. From what you say above, however, the relevant events prior to 1976 weren't called "US Open."

Anyone have any good tournament information from the early 70's?
Then Behrman's events started in 1976.