Part of the first generation Shimano 105 group, commonly known as the Golden Arrow group.  Also available in the (rare?) GS long cage version.  Servo-Panta Mechanism, Hatch-Plate Mechanism.  Max front chain ring difference spec: 13T.   

This is a very interesting and probably unique example of the Shimano 105 Golden Arrow derailleur.  It was found on a Miyata 310 being sold on eBay with a partial Shimano 105 group (non-Shimano crank).  The jockey cage is also a slightly different style than then standard found on other 105 Golden Arrow deraileurs, specifically around the lower jockey wheel.  One source states that this derailleur was produced in 1986, just prior to the introduction of the next generation 105 group.