I just checked out 106 on my 722. It immediately came up as set for "gray bar," and Andy Griffith looks as good as he did back in 1961. If all of a ch's content is in native SD I would see no need to waste bandwidth upconverting to HD. Other available settings include "normal," which appears to be exactly the same thing as "gray bar" other than the side "bars" are black. Indeed this is not letterboxing, as that references, typically, wider screen native content being shown in a 4:3 framing, with the "bars" at top and bottom rather than at the sides. Leaves the picture in a horizontal "slot" across the full width of screen, resembling the horizontal slot in a house door for mail drop. I guess this could be called "windowboxing," but I've also heard that term used when there are bars both at the sides and top/bottom of picture, such as when wider screen content is converted to 4:3 with letterboxing and then shown on a widescreen TV, adding side bars.
The other aspect settings are "stretch", "partial zoom" and "zoom". I would never attempt to use those settings on 4:3 native content like older TV. Oh, yes, they all 3 serve to fill the modern 16:9 screen from side to side but radically distort and/or crop the image. "Stretch" puts all of the image on widescreen but horribly distorted so that everything is shorter & fatter. No thanks, and all it further does is to constantly remind me that I'm not in the 20th century anymore, which kind of defeats the whole purpose of watching classic TV. "Zoom" both enlarges the picture and crops off top & bottom to fill the 16:9 screen, leaving both an incomplete image and reduced sharpness. Another big "no". And "partial zoom" appears to combine in stretching along with enlarging and cropping for the best, er, worst, of both worlds- noope...
The "thing," as I see it, is that some people are sensitive to getting "a" picture across the entirety of their 16:9 HDTV screen, over and above any other considerations, such as it being a good picture that displays all of the native image without distortion. Me, I love those sidebars, as they indicate to me that I'm getting the original, uncropped, undistorted movie or TV show. If today's huge TVs still aren't huge enough to watch 4:3 content with sidebars and have plenty of picture size, I don't know how I can help further. Some just have that need to always fill the screen and will happily watch a crap picture as long as it does so.