Old Sigma 80-200mm

Day off and I wasn't going anywhere as the pain in my back is almost unbearable at times. Its not just one place or type of pain either. Anyway I cant be bothered to moan about it anymore.

This little beauty, I found in the loft while looking for a piece of equipment I bought in 2016. The equipment I was looking for is nowhere to be found so I must have done my cardinal sin of selling it or giving it away as I have searched all corners of the house and loft! 
So I had no idea I even had this lens it was close to my old Nikon 601 film camera. I brought it down and cleaned the lens which had a smoky blue haze on it. Hooked it up to the D7100 and snapped a few shots. I am actually quite impressed with it as its so old!! It has to have been in loft at least 15 years and I cant think if it was mine or my late brothers old lens. It cant have been my Dads as he used to have Olympus equipment.
In the extras you'll see a couple of the pics from this lens at F8 (love the manual focus dials etc on this) at 320 ISO and at full 200mm zoom.

It must be over 30 years old. Does anyone know how I can accurately age it?

Found out it was manufactured in 1981. 40 years old! Not bad for it age!

Draco