Your “Pocket Vernier Caliper” is first shown in a Darling, Brown & Sharpe single sheet advertising circular dated January 1, 1867. The same tool cut and description is also in the 1867, 1868, 1869, 1870, 1875, and 1877 Darling, Brown & Sharpe catalogs. By 1881, this tool was no longer offered for sale. It was replaced in the company’s product line by the “Improved Pocket Vernier Caliper, which was of a different design and first shown in the 1877 catalog. So your example dates to between 1867 and 1880. Also, as “rivett608” points out, this tool was made with the option of having an “adjusting screw,” but the “adjusting screw” model was not introduced until 1870.
Your “Pocket Vernier Caliper” was designed by Samuel Darling while he was working in Bangor, Maine in the tool making partnership of Darling & Schwartz. When Darling dissolved his business and joined with Joseph R. Brown and Lucian Sharpe in 1866 to form the Darling, Brown & Sharpe company, most of the Darling & Schwartz product line was incorporated into the new company’s offering of small precision tools. The 1867, 1868, and 1869 D. B. & S catalogs all include a full page of tool listings that is almost identical to an advertising circular used by Darling & Schwartz. Your “Hardened Pocket Vernier Caliper” is listed on this page, selling for $6 and for $7 if in a “Morocco Case.” There are no tool cuts on this page, only descriptive text. In addition to this page, all three catalogs have on another page a tool cut showing front and back views and descriptive text which “rivett608” has already described.