Joke aside, your last statement is probably right. As you can read and discover at many places on this forum or elsewhere, the soprano sax in general has remained in some kind of backlog behind alto and tenor in it's development until the 70s. The MkVI clearly belongs to "before", and therefore is touchier when it comes to intonation (among other characteristics).
Adding to that Selmer's still very handmade nature by then, specially on the "low volume" horns, each individual specimen will have it's own ups and downs. So you really have to try each specific horn and discover by yourself how it plays.
And be prepared, this is no joke, the MkVI is not an easy soprano, specially compared to modern "post Yamaha 62" horns.
As someone mentioned in another thread, the MKVI will be more appreciated by a dedicated soprano player who will grow into it, than by a "doubler", who will find it much easier on a japanese or even taiwanese horn.