I know very little about Erlotti. My best guesses are as follows:

• Erlotti were based in Milan.
• During the years of the Second World War (and possibly slightly before and after it) Erlotti produced Fichtel & Sachs derailleurs under license - usually branded 'Torpedo'.
• After the end of the Second World War Erlotti may have produced this item - their very own development of the Fichtel & Sachs design.

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).

• Derailleur brands: Erlotti
• Country: Italy
• Date of introduction: 1947?
• Date of this example: unknown
• Model no.: unknown
• Weight: 234g including hanger plate
• Maximum cog: 22 teeth?
• Total capacity: unknown
• Pulley centre to centre: n/a
• Index compatibility: friction
• Chain width: 1/8”
• Logic: low normal
• B pivot: sprung
• P pivot: none
• Materials: steel