Here i attached the side view for mk1 case and mk2 case:

mk1

mk2

in the second the lugs are much thinner, so the cases are very different, not only for the back case,
that has the gasket outside the screw in mk1 and inside the screw in mk2.

Looking at some watches recently sold, i can say that the transition from one case to the other was not between 80000 and 90000 but between 60000 and 65000.

This watch is doubtless a second-ex case, second-ex dial, and the serial is 64271:

This is the only watch i've seen in the serial range 60000 - 84000 so this period is still obscure to me...

The transition between 2nd ex dial and 3rd ex dial should have happened to me between 85000 and 86000:
the earliest watch with 3rd ex dial i've ever seen is 85342, while the last with 2nd ex dial is 85349