NOS SIMPLEX Demultiplicator + Cable Guide Ref. 3637 L: Dual Downtube Clamp
This gizmo is used (primarily) with friction bar end shifters to make the rear shifter less "sensitive" to input and easier to shift smoothly. It changes the shift ratio to allow for bigger lever movements per shift. The left side is a simple tunnel guide/stop for your front derailleur. It was also recommended for tandems.
This straight band clamp with the Simplex moniker was used in the 1970's and 1980's. The earlier 1960's version had a pointed band with the S logo.
New old stock, no package