This Idéal derailleur is pleasingly elegant. In the style of the times it is a sliding rod mechanism, but its genius is that the sliding rod slopes down at an angle, to allow the derailleur to follow the profile of the freewheel. In this way it maintains a relatively constant chain gap - prefiguring SunTour's 1960s slant parallelogram that is still influential today.
I am aware of two variants of this design:
I do not know Idéal's model names for these variants - you could guess 'Course' and 'Tourisme' but you might be wrong.
Finally, I would note that the pulley wheels and their associated nuts and chain catchers on these Idéal derailleurs are extremely similar to those on Super Rapid derailleurs. I don't know if Idéal manufactured the Super Rapid derailleurs or vice versa or if both brands shared a third party supplier of these parts.
This is an example of the 'racy' single pulley wheel model. In this case the main sliding rod components are manufactured from aluminium - which is lighter, but also more fragile, than bronze. In the absence of anodising, the aluminium parts are finished in silver paint - which I believe is original.
This derailleur is astoundingly light, at 166g including the large steel mounting arm. However, it is also noticeable that the sliding mechanism on this aluminium version does not operate as smoothly as on the bronze versions. What goes around comes around.