Appears to be an early 1960s Schwinn Paramount. I forgot to jot down serial number to check. The details point to early 1960s-boxlining, early Campagnolo headset, raised pedal lip Campagnolo cranks, Campy dropouts with hole for Gran Sport spring.
Set up as a fixed gear with Campagnolo inch pitch chainring, 1950s Campagnolo rear hub, and block chain.
The current owner bought the bicycle as Sheldon Brown's personal bike. I don't know if he has the info to back up the Provenance, but it should be that difficult, as Sheldon lived a lot online, and continues to do so.