Your machine is in the Hoover U8100 series. They made these in a few different designations, like V-2, Dual V, VS, and Savvy. Most were sold as bagless but they offered "convertible" sets whereby the user could opt for bagless or bagged function, getting a separate dust bin for each. The bagged function is only supposed to be done with expensive HEPA Y series bags; they are kind of weird in that you install the bag into the dirt bin which installs cassette-like just like the bagless bin. I don't know the exact time line for the various iterations, but my observations on production dates in the serial numbers show the V2 first, then Dual V and VS, and lastly Savvy.
Within the U8100 series, you have the four agitator design and the single agitator design. I believe the single agitator model came after the quad agitator versions. Hoover had lots of trouble with these as you mentioned and the single agitator may have been their attempt at a solution. The higher model numbers in the U8100 series are where the single agitator versions are concentrated and these are marked Savvy.
The the Dual V/V2 and the Savvy incorporate Windtunnel designs, the distinction being the vacuum channeling around the agitator(s) which we've belabored so lately in another thread. Both the quad and single roll design types have dual suction in the floor nozzle, one port on each side and the Windtunnel theory has been incorporated into this situation.
One of the differences between the earlier V2 and the later models is the clear floor nozzle agitator housing (with the upper half of the vacuum ducts in it.) The later versions had a kind of smoke color part, rather than clear. This could be because the clear shows dust coating even with normal use; smoke tends to hide this.
This same part is one of the weak points on the U8100 series. The plastic doesn't last; namely, the screw holes tend to crack out and break away.
This machine is a fairly unconventional design. The vacuum ducts in the floor nozzle are fairly long and narrow; in the back of the nozzle, they make a 90 degree turn at the point where they join and are articulated to the handle. This sharp turn invites clogging, but fortunately it's fairly easy to take this apart to clean. Some of these machines that I've worked on had clogs in the floor nozzle ducts from dust-bunny accumulations that snow-balled into the entire passage being packed full.
The quad agitators are driven by a timed gear train. In the gear train are nylon gears which drive metal shafts to the agitators in pairs. Having the complexity of four opposite rotating agitators of fairly small size invites some problems. First, they seem to get stringy debris wound around them fairly easily. When this happens, they resist rotation against each other and jam up. When they jam, it can turn the metal shaft inside a nylon gear. Then the timing of the agitators is thrown off, and the bristles may enmesh with each other engendering more friction. In the worst case, the metal shaft strips and won't rotate one pair of agitators at all. The gear train replacement costs the better part of $75, but you can usually take it apart and fix it. Epoxy will re-set the metal shaft in a gear, but make sure you get the position of the shaft properly situated as to gear concentricity just before the epoxy sets hard.
The agitators must be installed in a certain way to rotate properly and not have bristles contact an adjacent agitator. The agitators work in diagonally opposed pairs. Hoover marks the agitators as lefts and rights. Properly installed, the agitator on the front right is a "right," and the front left is a "left." Then the agitator in the right rear position is a "left," and the one in the left rear position is a "right." The metal drive shafts in the gear train have flats on them and should be at 90 degrees to each other. If not, one has slipped.
In the pictures of the agitators shown in the post above, it appears that they are installed improperly as it looks like bristles are enmeshed. This shouldn't be the case; Each agitator should rotate without ever touching the bristles of the other one paired with it.
The later models of the Savvy have only a single agitator. As ever with this series, they are unconventional. The machine retains the separate motor for the agitator (of necessity with this design). The agitator is run from the motor via what Hoover calls a timing belt, which is cogged. It matches up with a corresponding cogged surface in the center of the agitator. This last design feature is a weak point on the single roll Savvys. The cogs on the roll are put on as a plastic sleeve over a wooden roll. The plastic cracks, the timing gets off and wrecks the belt.
The dark green color of your machine is very common. They also used a medium green, and later many Savvys are seen in a light grey-green color. They made some in red, blue, black but these are not nearly as common.
Another variation is a model without the turbo hand tool; just the short crevice tool, upholstery tool and dusting brush. I've got several of the U8100's on hand at present. These things are not the best Hoovers, Heaven knows but the unconventional design interests me. I've sold them, have several ready to sell, but I don't push them. Once in a while, someone will take an interest and they might say something like, "Gee, look at all those brushes -- I'll bet that picks up like crazy. I want it." I always point out the relative complexity of the machine as against something more common. If time counts as anything, I don't think I make much money on the four roll 8100's.
Below is how the brushes should look when set up and timed properly.
