I agree they are rather quiet and smooth, but I don't think you should exaggerate this. A while ago I had 2 mowers: a Victa Impala 4 with a 1976 Briggs 92908, and a Rover 200ES with a 1983 Briggs 92508. Both were very smooth, easy to start, and all that good stuff. The Victa was the nicer mower, due to a comfort handle with a good vibration isolation mechanism. The Rover was much quieter, because it had a large muffler where the Victa just had a little pot muffler. I liked both mowers, until I got an early Honda HR194 (4 height levers & bar blade, made in the mid 1980s). The Honda was so much quieter and smoother there was no comparison. Then I got a Honda HRU195 from about 2007. It was even quieter and smoother than the HR194: I could mow with the engine idling, and the loudest sound I could hear was the grass rushing around the grass scroll into the catcher. Of course mowing with the engine idling was not practical, since the cut wasn't totally smooth and the catcher chute tended to clog, but it sounded great.
So, that Rover is smooth and quiet because the version of the Briggs 92508 made for Rover was a smooth quiet engine with a good muffler. However it was not state of the art even for the mid 1980s: the Honda GXV120 was in a higher class altogether. I loved my Briggs mowers when I had them, because they were far and away the best I'd ever had up until then. But now I'm spoiled - I've walked right past similar Rovers on nature strips.
