Rear Caliper, Hollow Piston, which way in?

Replaced a trashed rear rotor on this 2001 KDX200. Rear caliper was frozen up. Disassembled and am rebuilding it with new seals and a very thorough clean up. Have rebuilt many older Kawi KZ1000 calipers before, and changed pads on modern bikes...

This KDX caliper piston is hollow. The hollow side is facing out as it stands but i don't know if that is factory or someone did it backwards. The old KZ's had solid pistons with one face clearly going toward the pads. I have the service manual but it looks like they show the flat faced / slightly dished out side facing out of the caliper and up against the back of the brake pad.

The kawasaki.com parts fische gives no particular insight, drawing is not detailed enough.

I could see it being either way.

My guess is hollow side is in the caliper full of fluid to help cooling with the flat slightly dished out face out toward the back of the pads. But I could see the hollow side being out to keep the hot pads as far away from the brake fluid as possible. Either way the caliper is gonna gain heat from braking.

So which way shall I reassemble? Hollow side in or out?

edit to fix typo, hurriedly typed it at work today...