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
2014 Additions: 1985 Trek 560, 1992 Trek Multitrack 700 (my 2nd), 1994 Trek Carbon 2200, Peugeot PX-10, 1981 Schwinn Voyager, 1989 Bridgestone RB-1