OK, I have found an answer in Nimmergut's : since 1867 and military convention with Prussia, the long service award for officers of saxony Altenburg, Meiningen and Cobourg-Gotha was the prussian one.
OK : 2nd class, round medal, bronze gilted, green ribbon, but for how many years ?
Now : 1st class = 20 years but no pictures found... same medal ? Another metal (silvered maybe ?). Probably same ribbon...
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