Shimano 600 AX II (6300) derailleur

Perhaps more than for any Shimano 600/Ultegra generation before or since, the Shimano 600 AX (6300) was very obviously a contemporary Shimano Dura-Ace model - but made cheaper. The finish is less polished, the silky anodising is nowhere to be seen and the p-pivot bolt has an Allen key on the outside (rather than hidden on the inside) - but from 2 metres away you would be hard pushed to spot the difference. Weirdly, the less-expensive Shimano 600 AX (6300) is just ever-so-slightly lighter than the pricier Shimano Dura-Ace AX (7300). However when you take into account the difference in the weight of your wallet the anomally probably disappears.

One notable difference is that, whereas, on the Shimano Dura-Ace AX (7300), the designers kept revising the exact shape of the outer pulley cage plate at the guide pulley, on the Shimano 600 AX (6300) they left well alone - and the same shape was used throughout its (admittedly brief) product life. Despite this, Shimano still managed to release four differing versions in just 3 years:

The first version (1981?) had the adjustment screw for the number of speeds mounted on a separate plate attached to the the b-knuckle and acting the cable guide.
The second version (later 1981) saw the adjustment screw for the number of speeds moved onto the b-knuckle.
The third version (1982) saw the inner pulley cage enlarged to completely enclose the guide pulley.
Finally, for the fourth version (1983) Shimano reshaped the 'ramp' behing the b-knuckle, removing the steps - and thereby removing the indexing from the derailleur. This version was rechristened the Shimano 600 AX II (6300). Why its number did not get incremented to, say, 6310 is anyone's guess.

This is a clean, unused, example of a Shimano 600 AX II (6300). It is similar to the third style of Shimano 600 AX (6300), except that the steel ramp at the b-knuckle is smooth and does not have indexing 'steps'. Some of its features are:

The adjustment screw to set range (5, 6 or 7-speed) is integrated into the b-knuckle.
It has a smooth steel ramp on the inside of the b-knuckle that controls the angle of the derailleur, but is not indexed.
The inner pulley cage plate has a large chain pusher area completely surrounding the guide pulley.
The inner pulley cage plate has a chain-nudging indent between the guide pulley and the tension pulley.
The two letter code 'FG' is cast into the inner pulley cage plate. There are also two other, more normal, stamped letters - in this case 'HB'. I believe that 'HB' is the Shmiano two letter date code. It is unclear what the 'FG' refers to.
The pulley wheels are blue.