To the best of my knowledge they are WP50 extremes. They are date stamped June 98 on the casting and came off a 98 LC4 640 Adventure or possibly a 620 see pic from the KTM parts catalogue below.
Here is a pic of the only information cast onto the sliders.
Good to hear that your bike handles the 320 rotor well, that may be the way I have to go. I think the standard 260mm rotor just won't be enough.
So far I have seen 3 variations on the WP50 fork caliper mountings.
The type depicted below that I think ran to 1997, it has 60mm spaced caliper mounting points. I have these
The later 1998 onwards version that is by far the most common with a 90mm spacing between the caliper mounting points
What looks like an older ie pre 97 version that I have only seen once, posted somewhere on this forum.