GMT 2 16760 query

[snorky]

Hello everyone

newbie to the forum ,but not a newbie to Rolex

sorry to start with a query, but I recently got a GMT2 16760 from an established . not official rolex second hand dealer

as a precaaution , i took the bracelet off today to check the numbers

Top lug: Usual Rolex design text & 16760 below, so I know its 82-88 era, probabaly 84 I think
Bottom Lug - serial number 439****

the serial suggests case date of 1974/5

However the top lug suggests it can only be '82 onwards- which is what i expected
Everything checks out - bracelet etc looks right

Runs well, has the GMT 2 jump facility on the first notch when pulled out, so is def a GMT2

Any clues as to why the case has this apparent discrepancy between the serial and the model number?

Obviously I have asked the seller and am awaiting response. I paid by CC, so can always raise an issue should this implode

any ideas or comments welcome

cheers

Scott