This is an antique precision tool called an Outside Caliper.  It is 8" size or about 9.25" long overall.  It is marked by the maker "THE L.S.S. CO. ATHOL MASS." on one side and "PATENTED JUNE 2  1885".  This is a FAY PATTERN outside spring caliper in very good working order with a good strong spring at the top, good point and good working threads.  A very nice, useable antique tool overall!  By a well known quality tool maker!