In short, early SNES decks have a BA6592F encoder, which spits out instead of clamped RGB, YPbPR!
This VERY ELUSIVE datasheet was helpful:
Here is a writeup:
And a video:
it would be very nice to somehow get additional BA6592F encoders and simply put them into other systems instead of paying so much money and putting so much time into getting YPbPr into MDs and N64s and such.
This is great for us silly crazy yankee Americans where YPbPr reigns supreme while 15khz RGB is hard to utilize.