I suspect the top of the trailer steerer has a crown race of sorts (or at least a machined step to act as a fixed spacer), and the top cap is just to hold the dyanamo charger thingy on
https://www.lafraisecycles.com/remistrailer
I did look at all of those photos but it's not really any clearer though I think you're probably right on the hard stop at the top.
@PhilDAS started
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I suspect the top of the trailer steerer has a crown race of sorts (or at least a machined step to act as a fixed spacer), and the top cap is just to hold the dyanamo charger thingy on
https://www.lafraisecycles.com/remistrailer