We talked about the foam over at the Lansing Heritage forum and some of us thought it was for dispersion control, some foams can act as a lens.
Back in the 70s and 80s EV used a foam lens that covered a large dome tweeter (Superdome) leaving a small hole in the center, it looked like a foam bagel. The lower frequencies from the dome passed through the foam but they higher frequencies came only through the small hole thus making for wider dispersion than a dome that size would normally have. This device allowed EV to use a larger than normal dome (and thus get higher output and a lower crossover than most domes) and still have wide dispersion.
The working of the lens is explained in this sheet from the EV archives.
http://archives.telex.com/archives/E...%20C%20EDS.pdf