I know very little about Erlotti. My best guesses are as follows:
It is quite a tidy object, with a pleasant quality of finish and a covered spring at the b-pivot. There is one strange feature that I don't quite understand. The small chain pusher plate at the pulley wheel is mounted in-board of the pulley. The Fichtel & Sachs design has this plate is mounted out-board of the pulley. This plate has a curved surface that is clearly intended to contact the chain. The Fichtel & Sachs layout provides a push to the chain when changing from smaller sprocket to larger sprockets (which seems logical). This Erlotti uses the plate to push the chain from the bigger sprockets to the smaller sprockets (which seems illogical and probably unnecessary).