I think this is a very well-used example of an early drop-out mounting SunTour 888 Wide derailleur. It has a bijou enamelled name plate but the lush chrome so evident in my ‘2nd style’ example has suffered somewhat. I have seen an extremely similar version of this derailleur with the word ‘Iwai’ on its extremely similar enamelled name plate. Iwai was the company that initially made derailleurs for SunTour, but it went bankrupt in 1958 and SunTour brought derailleur manufacturing in house. I think this example was manfactured by Iwai.
The underlying design remains a version of a Huret Competition - albeit with very ‘touring’ dimensions and the top pulley concentric with the sprung pivot. The sprung pivot is a full 95mm from the hub axle centre.
Overall, it sets the tone for four decades of SunTour long cage derailleurs; it is strong, simple and well made.
This example varies from my ‘2nd style’ example by having ‘Sun Tour’ engraved on the outer pulley cage plate in rather dashing cursive script.