Many vintage Klipsch speakers, for example, don't have internal damping of any kind. They were designed not to need it. I've got (6x) Heresy Industrials in the shop I'm writing from, and none of them have any damping at all. (Klipschorns were used in The Loft sound system if I remember right.)
There is an apocryphal story of someone being kicked out of Paul Klipsch's house in Arkansas because they asked if internal damping might improve the sound. As the story goes, he was outraged that they would think he hadn't considered something like that in his designs.
Furthermore, I'm about to build some cabs for some vintage full-range horns (Altec 511), but those will actually work fine mounted to a board, with maybe a little modelling clay or similar around the throat.
So, it is quite possible that it's not there by design. If you really get stuck, ask for Marty McCann at Peavey. He can help you. They're good people in my experience, despite moving their manufacturing operations from Meridian, MS to China.