I thought it reminded me of a "Boost Bottle" from my 70`s two stroke dirt days, so that’s what i searched not knowing boost bottles go on four strokes as well.
So this has been translated,,,,,
", the mix The fuel at the closure of the membrane is not again carried out by the carburettor, the resulting negative pressure only goes back to the Boost bottle, which is mounted between the carburettor and the membrane. reduces the resonant reflection air-fuel mixture, the blend is not enriched. Boost Bottle performance results are: decrease fuel consumption, better response to movement of the lever in the lower speed range and increases performance. effect is most noticeable at higher carburettors (from 19mm), which have a tendency to enrich the mixture " Apparently, only a few hundred pieces XT600 it is equipped with, rather than as a curiosity and study how the market would react. Larger effects of gas box was not so ceased to mount and promote this solution. The man pages in vain to look for information about the item and have not met that was available in the catalogues of spare parts. So XT is exotic and if damaged it can confidently replace the usual connectors without the gas box.
So its rare if you have one & not available from Yamaha in the parts bin.