A small gap ist normal is normal but it looks slightly larger in your case but would still not explain the height difference.That the adapter/reservoir usually needs to be slightly pushed into the correct position is also normal but if it requires some force, there must be something wrong.
Just to be sure: There is only one normal o-ring with a height of 3 mm (55 mm ID) inside the adapter that faces towards the pump?
I can not see the lower part of the pump but assume it is standing directly on the mount with its plastic plate?
Do you maybe also have a photo where the hole of the adapter and mount is shown a bit from the side to get a better idea of the distance between both of them?
Hey - thanks for the reply. Yup, there's only the one single O-ring. It looks the right size.
I've taken a bunch of other photos that will hopefully be helpful.
Here you can see the whole height, unscrewed:
If it's not standing on the platform, the holes would be fine:
Side by side as requested, kind of hard to see the misalignment though as it's when it's ON the bracket it's wonky:
A more helpful photo:
This is what I end up with when it's screwed together... the leaning tower of Ultitube
I'm pretty sure that it's the gap (as per my previous 'bonus photo') that's causing the problem.... yeah, I'm out of ideas. Looks like a return?