Yes, you can use ANY BELT....even the knurled belts....on the older models. The ONLY reason Kirby says to use the smooth belts on the pre-Generation models is that they didn't have ball-bearing brushrolls back then, they were still using sleeve bearing brushrolls. So the knurled belt is supposedly, according to them, harder on the brushroll bearings. However, if you have replaced your brushroll within the last 10 years, it will be a ball bearing brushroll, in which case you will be perfectly fine if you use the knurled belts.
Rob