The early Shimano 600 derailleurs were revolutionary in a number of ways. They were part of a full groupset - a new phenomenon for a mid level price point, they were designed for Campagnolo drop-outs (gone was the homage to Simplex), and most important of all, they were well finished and looked ruthlessly composed and modern. I can well remember marvelling at the sheer glamour of mid-range bikes kitted out in the full 600 groupset - they looked so much better than their peers clad in a mishmash of Weinmann centre pulls, SR cranks and SunTour V series gears.
This example is a long pulley cage, black, version of the second generation which have 6100 model numbers. The distinguishing feature is that the ‘600’ on the outer parallelogram plate is in small script - I have never been able to detect if there were any more substantive differences.
All in all, a pretty damn fine derailleur - even if it did not change quite as well over a wide ratio freewheel as a SunTour Vx GT.
The 6100 series of Shimano’s 600 groupset was the first to sell in quantity in Britain. It feels like an old friend!