Do the plat keys only drop off the x2's? Not gonna take a chance on the 800p dirty keys on the broker lol
if i understand well your question you can drop it from any boss inside the fabled cleft of rujark and it can be turn into a rusty, gold, platinum or silver key. Plat one is the rarest one to be converted in so.... go farm !
I got one off the gatekeeper right at the beginning of the 2nd floor. So yes, any named should drop them (tho I'm not sure about 1st floor).