That's great stuff, Lau! And what a great find!  

The two plates showing the same street are fantastic talking points for a collection, but I don't think it's surprising to find two typewriter related businesses located close to each other. If the Netherlands is anything like Canada, it's not uncommon for similar retail segments to be clustered; for example, in downtown Toronto there are numerous sneaker stores (don't ask) within a two-block radius of each other. 

What's more interesting to me is the sticker to the left of the plate on the Smith-Corona. It seems that it might be a clue as to why the 'therefore' slug (I've never seen one of these in person before) is on the Super. The sticker looks to be as old as the machine; do you recognize it?

With respect to the Dutch keyboard/characters, I think they're factory original. You can forget about that brochure you linked to: For one, it was meant for the United States market, and two, it's just a listing of the more "popular" keyboards, not a complete catalog of what Smith-Corona had available. 

I'm sure Smith-Corona had a Dutch keyboard, but as you can attest from the difficulty of finding such machines it wasn't a high volume item. Additionally, foreign language keyboards were also sold domestically and purchased by those who needed to be able to type in another language, something that is so easy to do today using ASCII codes.

Laurenz van Gaalen wrote:

My theory about the therefore-sign is that this machine was used by someone who was involved with freemasonry. If the therefore-sign was used for mathematical proof, I expect the because-sign (the same symbol but upside down) on the keyboard too.

Hmmmm. Looking at the keyboard again, it seems to be an iteration of an international typeset. You have three currency symbols, and accents for different languages. Rather than a mathematical use, I can see this keyboard being used by someone typing business correspondence, because the ∴ was apparently also used for shorthand. This seems far more likely to me than it being special ordered by a Freemason.
 

Laurenz van Gaalen wrote:

What puzzles me the most is why they added the therefore sign without the related because sign.

Wouldn't that support the theory that the  ∴  was used for shorthand and not math, especially given there aren't any other math-specific symbols (other than what you would normally find)?
 

I am pretty sure (as sure as the other explanations) that this three-dot character was ordered by an outdoorsman writer so that he (They are 91% male.) could indicate the locations of cairns in his trail descriptions. Mystery solved.