Hi Benjamin, according to this Belgian website the difference is in the distance between the two 1's (in stead of "large" and "small"):
1 = Smal - 0.6 mm
2 = Normal - 0.9 mm
3 = Wide - 1.2 mm
Of course, a difference of 0.3 or 0.6 mm is not easy to see.
There should be pictures on the site, but I don't see any. Maybe you have to register (free) and log in first to see the pictures.
The website is in Dutch, but I know you speak Dutch very well:
http://www.cbnu.be/index.php?topic=1461.0
There is also someone who says the second 1 is higher than the first 1:
http://www.cbnu.be/index.php?topic=181.0
And here is another website (in Dutch) with a big discussion about the may or may not existence of that variety, unfortunately again without visible pictures:
http://www.muntstukken.be/forum/index.php/topic,7511.0.html
Maybe these are the websites you are talking about in your own post and you already have seen these, but I hope not of course. Anyway, there is a lot of doubt about the existence of this variety and so far no proof of it. Hopefully some of our members can provide pictures with a clear difference.
https://en.numista.com/echanges/liste.php?piece=496: Several members have a large date in their swaplist, others have a small date there. But no one has a large and a small date in their swaplist. So it might be just a matter of interpretation which check box you tick. If you don't have both it's difficult to know which one you have and that's how mistakes happen.