I just bought a Dr. Grabow that was designed for a filter, and I havent had any problems smoking it without one. I will use a filter every once in a while depending on what tabacco I am using. It seems more a matter of taste so to speak.
It's a mortise.
Doc.
personally i hate em....also think those little widgets that new pipes come with in the end of the stem to stop hot gledes rising are a waste of time too!
Never have used filters at all in my pipes.