This is the only postmark that I have ever seen depicting a derailleur. It appears to celebrate the visit of the Vuelta España to La Coruña on the 24th of April 1989. The derailleur looks like a Campagnolo 980 - a rather low-end model to associate with such an exalted event.
As for the composer Tomás Bretón (the jolly dude depicted on the stamp), he looks like a man who plays a mean game of dominoes.
Browse associated derailleurs.