via this auction
"Further, this TR-909 is unique due to a little modification that, as far as I know, has never been seen elsewhere : the ride tune parameter range is roughly 5 times larger than on the original unit. The unit is capable to downpitch the ride tune way lower than the original, resulting in realy cool tonal possibilities for that ride. Basically, the original range is like now from 8 to 10 on the new knob, so from 1 to 8 the downpitched ride generates really cool metallic grainy textures"