Originally Posted by chain_whipped

Not exactly Osgear though of the era~
Have a circa 1920's "le Simplex" idler arm that sets directly under the crank, mounting clamp to the downtube with pivot and coil spring tensioner. Its designed to use with a chainstay mount derailleur. The arm is fairly elegant thin yet cast in heavy bronze with cross section 'T' for rigidity. Though originally had a lousy non-toothed, narrow and sharp pulley to ride the chain, plus the pivot where the spring wraps, it was very poor tolerance and sloppy. Did re-bush and changed the pulley to a more conventional looking toothed jockey wheel, yet still very much vintage and has deeper teeth. Problem solved.