Just a quick question, these nylon? ones from Knibbs have no instruction for fitting as they just come loose in a bag and I although there is nothing difficult in that, I see they have a flat surface one side and a rounded the other.
At first glance the rounded looks as it would seal better but it also looks like it has inner and outer leads to it so assume rounded edge goes in the recess and flat edge is the seal??

Thoughts?