1) At 9:43 when you are looking at the rotary switch, you can see two grey plastic clips that hold the white plastic ring. I purchased an used Fluke 113 that was "fully working", but arrived with a broken rotary knob. Obviously the seller was hoping I didn't notice when I was using it.
I discovered that one of plastic clips was broken. Replacements are available on ebay, but they are expensive.
The Fluke 170/80 series uses a different arrangement (metal shaft) as you can see in modemhead's blog at
http://mrmodemhead.com/blog/how-to-remove-the-knob-on-a-fluke-170-series-dmm/2) Regarding the display on your Fluke 115, some older Flukes like the original 80 series suffer from the fading digits. The elastometer degrades over time. For your 115 (at 11:14), try cleaning the pcb and zebra connector with q-tip dipped in isopropyl alcohol. It may not help at all, but it is worth a try?
I haven't heard/seen any reports of fading digits for the 115 at all.
3) In the pure speculation department, perhaps the 115 display isn't as good as the 17B because the 115 has a backlight? Perhaps the designers had to think about how the 115 display works with a backlight and thus choose material/glass, etc to work best with both scenarios?
In addition, perhaps the majority of 115 users use these multimeters outside in different conditions (sunny, dark, cold weather, hot weather) and perhaps the display is a "compromise" to work best in a variety of scenarios?
I doubt the designers thought the 115 would be an exclusively indoor used electronics multimeter.
4) If you are ever interested in selling your Fluke 115 for cheap, let me know!
5) Regarding S7 -> pretty cheeky!!!
