Though I know more about the older Sunn stuff, I know that Sunn used a more open, black cloth in these later models. The earlier cloth was similar to Fender's black/white/silver cloth. I dont know exactly what this black cloth was, but you could probably find something very similar at music stores that sell "genaric" grille cloth. I'll check my catalogues from the major manufactures to see if I find something specific.
The '60s Sunn grilles were held in place by "stick & ball white nylon fasteners that fit together with a "snap". I don't know about the later '70s versions.
Tom