I am not entirely sure what model you are speaking of? I did purchase two of the model 520M single stage ones and lok forward to using them in this upcoming season. I believe the site is www.simplicitymfg.com Hope the info helps.
I have a 8hp 28 inch simplicity 2 stage I bought new in 93, used 100% commercially til I got my toro single stage a couple years back, since then it only comes out for the big ones. never ever left me hangin, nothing ever broke which I couldnt ride the rest of the storm out with just things replaced like belts shoes and such. the impeller pulley has a clutch sort of like on a snowmobile so when you get into heavy stuff it steps up the rpms of the fan to really throw the stuff.