Hi krob,
sorry if I misled you, I've seen posts about some early XS650 carbs having vent lines that were confused with fuel lines but not on yours, eh?
And KQHB is an initialization for King/Queen/High Back. Every aftermarket seat manufacturer makes them for the Chopper guys and Yamaha made them as an option, too.
From the photos, yours looks like an aftermarket seat, as I remember the stock KQHB seat on my own (long gone) '79 Special, it's hump was far less extreme.
After I modified it by sawing it's tail off (stupid me, not realizing that seat's rarity and consequent high resale value) and replacing the torn original with a Saddlemens cover it looked just like a stock Special seat.