Hey, all from memory but should be correct.
Auto radiator has an oil cooler on the bottom for a.t. fluid, manual doesn't have cooler.
So can use auto on manual. Can't use manual on auto.
Ka24 and ca20 radiators are the same
No idea about r31 or u13... i think they would be different as the r31 is north south and the ka-de i think has a differnt water entry location for hoses. But unsure.
just looked at ebay and the u13 should fit.
the r31 has the outlet on the opposite side.