Thank you Noriyasu for taking your time to reply.
Apologies for not posting pictures earlier, this should make it a bit easier to see the issue. The SVX in question is MY1992 - as far as I know.
Here's the pump from the vehicle, with three outlets, one of them towards the oil cooler.
I've added a couple of markers to this photo to show a couple of features of the original pump.
Here's WPF006 from Aisin (it has three outlets too), with added markers. The roller in the Aisin pump is not high enough for the timing belt to fully latch on.
I'd need to see photos of the water pumps from this angle to check if they actually fit. I don't know if there's something odd in my SVX, or if the oil cooler has been retrofitted into it at some point.
Yahoo Japan does not ship goods to European Economic Area, so the link does not open for me.
GWSU-12B has one water outlet, we'd prefer a pump with at least two outlets. We can circumvent the need for the third outlet as long as the pump fits.
I'd need to get Monotaro's page translated somehow to order something from them. A genuine 21111AA033 does not seem to be available here, but I'll go check from the local Subaru dealer.