I got a nice forum member to send me jets from jetsrus.com. I live in Canada so they won't ship to me.
I got them yesterday and I noticed no K stamp on them. My heart sunk as I thought the order was placed wrong, but there is only one option for ordering for the roundtop carbs on their site.
They are aftermarket only!
The only place you'll be able to get genuine Keihin slow jets, OTHER than stock, would be through honda dealerships (if they are available).
FWIW, size 42s were stock on the CB400F model bikes.