the rule for telling the difference between a SS and a Nakajima is: look at the margin set buttons, SS button ends turn up, Nakajimas turn down. There’s not a lot of other obvious visual differences.
via Ted Munk at https://typewritersandbeer.wordpress.com/2020/02/24/silver-seiko-speedwriter-7200-and-stone-ruination-2-0/#comment-95