If it were mine, and I lived in Wisconsin, I'd take the bike to Wastyn's gambling that Oscar Jr. might have some recollection of it. It's special enough and recent enough to be memorable.

The Wastyn website has this Wastyn Special photo showing a single top tube (instead of twin laterals) from the head tube to the seat tube, then lateral mid stays from the seat tube to the dropouts. It appears to be similar vintage as the OP's chrome mixte. It looks the the S-A shifter cable is routed along the down tube to a pulley near the BB shell.