This is a coarse ground mustard, comparable to the mustards as they were made before steam engines facilitated the milling of the mustard seeds. Once the steam engines got introduced, the coarse ground mustard got forgotten...
The Tierenteyn Mustard à l’ancienne is not a regular mustard with some half-broken seeds in it: during production, a precise balance between the fine and coarse fraction is pursuit. This mustard is made of brown mustard seeds.
The Mustard à l’ancienne is a spicy mustard with some slow release effect of the flavour: while the flavour is still partly enclosed in the half-broken seeds, it is slowly released afterwards. Very tasty and often used in sauce-preparations.