I've seen both Shimano and Suntour models. The Somec looks to have a Shimano version. The mounts on each were slightly different shapes. It was an "aero" design popularized in the late 1980's
The SunTour version has a cam mechanism with an action that moves the front derailleur lever down as the rear derailleur lever is pulled, thus trimming the front derailleur as the rear moves the chain up the rear cogs. They used a name for it, "Syncromatic" or something similar.
John Siemsen SLO, CA