Ich setze mal den Text von Buttonarium in englischer (Google-)Übersetzung hier rein:
Purpose and dating
Buttons of this type, as presented above, are not listed in any of the known catalogs, but on the basis of many years of collecting information related to them, it can be assumed (at 99.9%) that it is a Prussian military button from the second half of the 18th century and the beginning of the 19th century. It is highly probable that buttons of this type were in force in the Prussian army from the time of the Seven Years' War (1756-1763).
These buttons are known with and without numbers.
The previously known number buttons with a plain bracket cross-section have numbers from 1 to 13. In the Prussian army from this period there were: 13 cuirassier regiments, 12 dragoons regiments and 10 hussar regiments [1] . So you can suspect that these buttons were part of the cavalry equipment. Unfortunately, it is not known today whether all the above-mentioned cavalry formations wore buttons of this type, but since they appear with the number 13, it must be assumed that at least cuirassiers wore them.
The previously known number buttons with square brackets have numbers from 1 to 54. It can therefore be suspected that these buttons were used by infantry, as there were as many as 60 Prussian infantry regiments [2] .
Smooth cup-shaped buttons with a square bracket cross-section are more common than those with numbers.
All the buttons discussed above are found mainly in the so-called Regained lands : from Masuria, through Warmia, Gdańsk Pomerania and Western Pomerania, including Kołobrzeg, Wolin, further through Greater Poland, the Lubusz Land, to Wrocław and Opole. As number buttons are quite a rare find, it should be hypothesized that they were either worn by a few soldiers (e.g. officers?) Or only in some specific place of the uniform.
Ich denke von einigen der genannten Fundregionen her kann man Österreicher mit Sicherheit ausschließen.
Viele Grüße
Jacza