7/8" width dates this to 1855-1935, widening to 1" after. Lack of 1/12" graduations dates it from 1874-1878.
According to "Boxwood & Ivory: Stanley Trandtional Rules, 1855-1975", it's between about 1909 and 1925, due to the graduations on the arm opposite the caliper. This seems to agree more with the origin statement.