Okay, so here are some pics of the wheels, the retaining clip, and a center cap. The wheel with the protruding portion is the front wheel, and recessed is the rear wheel. Thoughts?
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I think the thickness of your rims and the diameter of the center hole, where they meet the hub may be conflicting with the retainers.
Here's a photo of my rear, oem 'phone dial' rim. There's more space around the hub bore (the center hole in the rim) and the offset and/or thickness of the rim seems to be less than yours.
Another thing you're up against, is do you have the front retainers or the rear retainers? There's obviously a difference if there are a front and a back version, but I don't know what that difference is (diameter?, length of the 'fingers'?). So you'd need to try the retainers you have on the front and back (by jacking the van up and removing the wheels) to see what you have and what you'd need to get, assuming they'd fit with your rims at all.
The diameter of your rear center bore is nearly exactly the diameter of your hub, so IDK if you'll be able to get anything in there at all.
FWIW, here's a link to a Pajero forum discussing these components, and others commenting on how they sometimes dont work with aftermarket alloy rims.
If you can't get the ones you have to work, there are other solutions, like painting your hubs so they look more finished, or buying aftermarket center cap 'cups' that fit through the wheel hole from behind, and then get clamped in place when you bolt the wheel back on.
For example:
www.procompusa.com
But you might run into the same problem with the tight fit.
Here's a photo of a Delica with aftermarket rims with the hub 'cups', but I assume those caps came with those rims, so they work well together.
I guess you could also have your rims machined to increase the diameter of the hole a little bit, and that might accommodate the OEM or other options...
Anyway, if you find a solution, post it up here! Good luck.