Fantastic video.
1. That black one must be a more recent vintage because it doesn't have the iconic spherical keycaps that your defunct red one does. That also probably helps explain its immaculate condition. Nearly any Selectric old enough to have the classic spherical keycaps is going to be in much more used condition. Back then the Selectric was the Cadillac of typewriters and had not yet been supplanted by word processors or computers. By the time cylindrical keycaps began appearing on Selectrics we were already in the PC era and they no longer got such heavy use.
2. Selectric switches aren't loud per se. Most of that noise comes from the type element impacting the platen, not the key switch itself. I suspect that the Selectric switches are no louder than beam springs (without the solenoid), and that if you could eliminate all the electro-mechanical noise from the Selectric and isolate the switch sound alone, we'd find a surprisingly quiet switch mechanism.
