dont listen to Nicolas, i already have a 601 and a 611 sm600 from 1964.
they are great watches as are the 28x range of hand wind movements. i look forward to seeing the date of it.
BTW, watch this page as my watchmaker 'may' have found a source who will make display backs for sm600's and sm30's. the movements certainly deserve it.
here are mine for you to see if you havent already


best
Graham