I can't definitively answer your question, but it seems apparent from the various fora I've used over the years that Rolex tend to gradually introduce changes, so it seems quite possible to me that you have a 1980 model 'made in '79' - for example I know there are some matt dial subs that came fitted with Sapphire crystals brand new (I tend to lose track of the model numbers to be honest!) - in a nutshell I reckon your watch is OK :)