That is, you have two parts to mate and those thingies are supposed to help. You can find those in appliances, model kits, computer cases (to keep the bezel and the aluminum case proper together) and the like.
The projection fits into a hole, and then it's kept in place with a screw or bolt.
A VHS cassette:
Sometimes you have a flanged projection which doesn't require that, as it simply snaps in place sort of like this:
(Sorry about the poor draftsmanship, that crooked Christmas-tree-like monstrosity is supposed to be a flanged cylinder.)
What are they called? Dowels, projections, lugs don't seem correct. I'm writing a technical manual and I want to be extremely precise.