What really counts, when it comes to serial numbers ?

[andypandy]

Here we go ! My Gmtmaster has the serial number 2041*** , on looking up the year of manufacture it appears to be 1666 or 1967, depending what
list I use. The caseback is stamped 111 68. What have I got ? I have owned this watch for 27 years...bought from a friend who was the original purchaser. He had never had it serviced or repaired , so nobody had a chance to swap backs. Well what do you think , gentlemen ?