Peugeot: There was another thread about such, their bike looked to have a strange wheel size between 20 and 24 by my eye, and that was confirmed. Yours' looks closer to a more standard size.

Dahon: Sure enough! Yours' has an aft slot. Just weird that it still has a plastic shim. Probably dahon using up stock of them before going to aluminum. Glad to hear you are upgrading that to aluminum, there is no more important upgrade. ***EDIT: UNLESS, it has already been replaced with an aftermarket shim that is *black anodized*. Look at it close, it should be evident if aluminum or plastic.***

Additional: I think the shim top should be under the clamp top (turtleneck), but looking at it closely, it looks like it could work either way. Notably, the rim on the top of my aluminum shim, looks to be a hair smaller diameter than the seat tube outside diameter, so the clamp is squeezing down on the seat tube, not just the rim of the shim, there's a tiny gap around the rim of the shim. After clamping the new shim in place, shining a flashlight in that area, should tell if the same.