This mysterious long cage Shimano Dura-Ace derailleur appears in no Shimano catalogue that I have seen. However, I have seen two identical examples, which makes me think that it might possibly have been a Shimano product, rather than the creation of enthusiastic amateurs - but who knows?
Other spurious evidence is that the short cage model was originally released with the product code '9000' with no letters after it. But later the product code became '9000 SS'. The arrival of the 'SS' might be taken as implying that there could be a 'GS' version.
This particular model has the knuckles and parallelogram plates of a Shimano Dura-Ace 9000 SS, the pulley wheels and pulley pivot bolts of a Shimano Dura-Ace 9000 SS, even the pulley cage rotation stop of a Shimano Dura-Ace 9000S, but what looks like the pulley cage plates of Shimano Ultegra 6800 GS. Note that the pulley cage plates are aluminium, not carbon as on the Shimano Dura-Ace 9000 SS. Details like using the Dura-Ace pulley pivot bolts and pulley cage rotation stop are unusually 'correct' for an amateur conversion.