Lemania created a family of cam switching chronograph movements in the 1960s just as chronograph watches were becoming more popular in the mass market. The Lemania Calibre 1873 was part of a family of similar 12 ligne movements, running at 21,600 A/h and lacking modern features like hacking seconds or automatic winding. In 1968, Omega selected the Lemania 1873 as the basis for their Cal. 861, used in the Omega Speedmaster Professional line of watches. These feature a 30 minute chronograph counter at 3 00, a 12 hour counter at 6 00, and small seconds at 9 00.
The Omega 861 (and similar Cal. 863) differ from the Lemania base mainly in decoration. They used 17 jewels through 1992 before being reissued and updated with 18 jewels in 1993. Cal. 1861 and 1863 are rhodium plated. Cal. 863 and 1863 differ in decoration as well They feature more elaborate finishing for use in watches with display casebacks.