1st post to me looks like a K141 but with a piece of tape on that side for studio marking purposes.
If you look close that tape extends over the slightly fatter headband spot at the area just above the cups, which those have as I recall ( mine is in storage right now ).
Would kinda make sense since in that 3rd video they look to be putting a pair of AKG ( K240 ) on the girl to listen.
2nd post yes I think you are right! David Clark. I have seen a strictly audio version of their stuff, not common though. Only ever seen two for sale on auctions.
3rd I think it is those Koss adjustable-sound ones with the built-in lever. Forget if it adjusts low end or diffuse-field type of thing. I cannot recall what those are at the moment, and they are a dynamic..
Oh now I recall they are called the Koss Technician VFR.
VFR stands for Variable Frequency Response .
At least what I was thinking. There may be another thing they made that is similar???
Still rocking the baby stax ?
Had mine out for a walk just a bit ago.