It's too late to play with the camera, but some work with the calipers has confirmed exactly what Paolo has suggested:

• Case Number 49215 has lugs with a thickness of 3.3 millimeters, and
• Case Number 89311 has lugs with a thickness of 2.7 millimeters.

: 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