Mid-80's SR400, maybe?
Please post pictures of the drive side, including a close up of the rear derailleur. It does appear to have a Shimano 600 headset. My SWAG is a 1987 SR600. Get to a self service car wash fast and lube up the bike!
Life is like riding a bicycle. To keep your balance you must keep moving. Albert Einstein
2016 Additions: 1981 Miyata 1000, 1981 Schwinn Voyageur 11.8 chrome