I thought BHJ and Fludamper both made a perfrormance replcemant for the 6's (235 / 261, not sure about the 250 / 292 etc)?
Probably not what you are looking for, as they would be pricey, but should work better than the stock piece.
Probably doesn't help, but I hope it does.
Nex, I worked with BHJ on the 235 damper. Yes they do have them for sale - when we made it we specifically made it for a 2 groove application. Call Brian Clarke at BHJ and tell him I sent you.
