Well its a late 79 - 84, if theres no serial then its a 79 upto late November. Theres currently some debate regarding the serial numbers and if they actually mean anything. I was told straight from the horses mouth that they were all just random but a few people are now thinking otherwise.
Looking at your downtube decal I'd say your SN starts with a 4 or above but it may have been refurbed so it sould be lower. ( Rippers don't start with 0 , 1 , 8 or 9 )
anyway its a nice looking bike and I guess the owner knows what he has hence it hanging on display so be prepared to pay above £700
