This was the best Simplex derailleur of the period - I have ridden it and it works very well. I've stripped and cleaned and serviced it, so it's ready to go. The derailleur is near Devizes in Wiltshire if you'd like to take a look or collect.
Simplex SLJ 5000 rear derailleur would be an excellent fit for riders requiring a lightweight, effective, and reliable option from the late 1970s.
