If you are a bicycle mechanic, two of the classes of bikes that you will favour the least are kids bikes and folding bikes. Until recently, most of these benighted creatures were spot-welded, pressed-steel, horrors festooned with components that were the worst that (very little) money could buy. And as for BMX bikes - don't get me started.
But that, of course, makes jaded collectors particularly salivate over the 'special' derailleurs designed for 'the younger rider' or 'the smaller wheeled bicycle'. It is easy to become bored with the unrelenting loveliness and sophistication of Dura-Ace and Super Record. Away with those exquisitely minute servings of haute cuisine - bring on a proper plate of bangers and mash in a generous puddle of glutinous gravy.
Here is a modest selection of very modest derailleurs.
Shimano MJ (MJ10 SS) 1993
Zeus Alfa Junior 1970