Nice catch! This is most likely the prototype for the first generation of parallelogram rear mechs for the then upcoming 8 speed and ergo generation. I have the same as your production version for my new build.
Good point, in fact this model does appear in the 91 catalogue pared with non-Ergo 8sp set-ups. In 92 however the new Record group featured a different design for the r/mech and a variant of the above design was used for the Chorus and Athena Ergo 8sp groupsets.
Correct, its funny really as I also had this rear mech model first time round when it first came out in early 91...it seemed to only exist for this one year. Considering its small time window of existence, it seems like the odds were stacked against me getting the same type 21 years later.
Hang on!!! Cue twilight zone de da de de sounds......perhaps it is actually the identical one I used to have over 20 years ago