Quoted from arcademojo:Williams system 3 - 7 diver board. Was in a working game I picked up. Yeah it still works.
Looks like it could be High Temp silicone to me similar to link below. That would seal the board after a last ditch repair effort. I have used it in engine bay applications. Says its good to 650 degrees. Apparently its doing its job!
https://www.amazon.com/Permatex-81160-High-Temp-Silicone-Gasket/dp/B0002UEN1A