From your post it would seem the longarm passed proof in July of 1953 and wears the tube steel IZH(Izhevsk's) 50A steel, if stamp of 3 intertwined rings isn't present. May have "Simson" in scrip on the underside of the tubes. The pyramid stamp was one of Simson's trademarks.

Kind Regards,

Raimey
rse