This scan was sent to me by a correspondent. I have been told that this is a scan of an advert in the June 1911 edition of Le Cycliste. I can’t vouch for its provenance (it looks a touch ‘downloaded’), but I decided to include it simply because it is so difficult to find images of Prével d’Arlay’s derailleurs.
The derailleur shown is, I believe, the earlier Prével d’Arlay model, operated by a long rod with an ‘L’ shaped bend at the end which, when the rod was rotated, shoved the pulley cage sideways. Interestingly, this derailleur is specifically sold as a component that can be fitted to any bicycle - unlike the Terrot systems of the period.
Crudely counting teeth, I think that the freewheel shown might have 18T, 24T and 34T cogs. This is slightly different from the 18T, 24T and 36T described by Frank Berto.