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.