The Whispertone canister came out in later 1989 I believe. It was distinguished from the rest of the canisters by its long cord rewind bar and lack of vents between the bag cover and the motor cover, and the bumper which continued around the back side.
Linked is a catalog page from 1991 showing it. Each year, I believe there was only one Whispertone in the lineup, representing the top of the line machine. Typically they had variable speed controls and Power Mate Jr, but I don't believe there was ever a dirt sensor or a yellow model. Performance indicator lights and controls on the hose handle were also standard. Colors included dark gray, white, blue, hunter green, etc.
I think the Progressive canister came out about 1998 or so, and the Whispertone became a lesser model. We bought a hunter green Whispertone in 1999, and it did not have the variable speed control or performance indictor light (only a slider performance indicator). The manual shows these as options, so perhaps higher models still had them. The same body design was then used on lower end canisters, but did not have the sound insulating material, so they did not carry the Whispertone name, only those with the insulation did.
Around 2001 I think the round body Whispertones came out. They were still used with the 90s Panasonic style Power Mate. Again, the sound insulated models were named Whispertone, while the cheaper ones had no name. Colors included hunter green, blue, purple, etc.
Eventually the Whispertone name was dropped (don't know when) and the rounded canisters became BOL models. The last ones I recall were around 2008. The BOL was bright green and had the curved headlight Progressive Power Mate with no quick release, and the better model was blue and had quick release Power Mate and bag full indicator. These were then replaced by the current tangerine and aqua models.
http://i13.photobucket.com/albums/a251/farrini/sears921.jpg
