I identify this stamp as Norway #58 60o dark blue perf. 14.5 X 13.5
Size: 16 x 20mm and Wmk. 160 Post Horn. The watermark show easily with Zippo lighter fluid. On the back of the stamp there is "58" written with a pencil and there is a mark. Is this an expert mark? I search a stamp experts site but coudn't find a mark like that from a Norway stamp expert.

