I have two of them. hence, the question Still no enlightenment on the "M" ...

Mysterious ....

: Not so strange, I just thought that the number you suggested before
: was the watch in question - there's not a single batch :)

: I don't have any other serials for 315xxx to check other batches -
: it will be just over a year later than that serial, so 75/6 or
: thereabouts. But I do have calculators in 266xxx and 305xxx
: recorded too, so it's clear there were a number of batches
: produced, nothing to worry about.