As I understand it, a pin head oat is not rolled, just broken up. Mornflake then stone grind it to various textures.
Yes, stone grinding is an ancient technology for milling grains which has been replaced by higher speed steel rollers which mill more quickly but generating higher temperatures which changes the properties of the milled flour. With mustard grains for example, it runs so high that the sharpness of the mustard is lost and has to be added back in. For this they use radish which gives steel rolled mustard that sharp hit at the back of the nose. You don't get that with stoneground mustard. But, the millstones run much more slowly and it is simply not possible to produce the commercial amounts required today.
Old millstones litter the landscape in the places they were produced such as Stanage Edge. It's why that type of sandstone is called millstone grit.
The bigger ones started to be introduced with mechanisation, wind/water/steam engine. Before that grains were ground by hand using quern stones. Even these still lay on the ground, such as this iron age stone at nearby carl Wark hillfort:
Steel cut oats are not milled at all. They are literally sliced or chopped. Most of the processes affect the chemical composition and the particle size and the characteristics of the end product can vary trememdously. Grinding by stone is generally much kinder for all grains.
For photos of what different processes do to the oat grain, see this page on the
Whole Grains Council site.