Sorry if I was confusing - a habit formed may years ago, referring to room-temperature-vulcanizing, silicone based, compounds as "silastic" - I guess I never knew if that was an "official" term or not, but where I worked at the time, that was the common term.

If a black rubbery compound is present in the center of the webs then they are not welded in that area - the RTV is joining them and damping the "halves" of the horns. Altec incorporated the RTV in the late production of the 511B to damp the vibration everyone had recognized and complained about for years.