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Attached are schematics for two S600 balance chambers. Rev B is the older. I expect that the same balance chambers may be used on other models. So which one do you have on the R109s and G250Vs? The knob can be an indicator, the new knob is more tapered general with a chrome ring. If you remove the +- sticker then it is obvious as the new one has a Philips head screw.
The G250 and 156 do not use adjustable balance chambers.