This stamp was used by Zlatoust Weapon Factory between 1890's and 1917.
Source
http://www.imha.ru/1144531224-proizv...l#.VxPtK_krK01
Here are examples.
Minor variations are expected: no stamping device would survive forever.
The fact that somebody in today's Russia produced a copy proves only one thing: Russian collectors are paranoid not for nothing; they are bombarded with fakes.