When I was 16 about 30 years ago I bought the exact same set with the same dragon image and everything. It's like some kind of gray plastic tubing, probably conduit or PVC. It has maybe 1/2"ID and 1/8" wall thickness which when covered with the foam gives you a perfect diameter. It was dense open cell foam, but the surface is smooth so it feels more like smooth pleather than foam. The result is about as perfect of a finish and feel you could ask for. The gold dragon image wears off fast, but the foam stays perfect. You might want to smooth the inside edge of the tube if it isn't already so it doesn't start wearing into the rope.
They're very light as you can imagine, but you can still get them moving pretty fast before you feel the air slowing them down. If you want to add weight there's plenty of room, maybe put a wood dowel inside?

I live in California so the cops took mine several years later. It's a serious felony here!! I spent the night in jail, $5K bail and everything. The only thing that saved me is when I explained to the judge that they were so light and soft that they couldn't even give you a bloody nose, and I asked the police to show them to the judge. But somehow they couldn't find them!!! No weapon no crime! Talk about a guardian angel! A felony weapons conviction for nunchucks is a 25K fine and 5 YEARS in prison!! That would've ruined my life. An ex-con w/ a felony weapons conviction would've destroyed any chance of white collar jobs or good blue ones, and god only knows what else. Plus I'd never be allowed to own, or even touch a gun ever again. Since I've been a gun nut since I was 8, that would've killed me.
Sorry for the long story, but now you know what they're like and made of, and that they can ruin a mans life.

Robert