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
2015 Additions: 2001 Eddy Merckx AX Titanium, 1981 Peugeot PKN10, 1987 Centurion Ironman Expert, Raleigh Super Course MKII