Doug,
This is a picture of the old emulsion tube from my carb. I replaced it with an exact copy from the rebuild kit.



The tube is two different diameters on either side of the shoulder, allowing it to go in only one way. The shoulder bottoms out against the carb casting and projects slightly into the venturi. I set it with a wooden dowel and am reasonable sure it's bottomed. I don't see where an o-ring would go.

Eric