Marlin Glenfield Model 60

My newest addition...a Glenfield Model 60 .22 long rifle semi-auto with an under barrel 17 round tubular magazine manufactured between 1966 and 1982...in almost pristine condition. It's topped off with a vintage K-Mart 4x15 scope that will be replaced with something a little easier to see through...

 

So...who here can help me nail down the exact year of manufacture... !?

serial # 206266xx

100-20 (1st two digits) = 1980 ?

 

Thanks guys...