I just found a video that was newly posted to YouTube. It is quite old, as is evidenced by how young Sebata-san and Katayama-san both are in the video. It says they are fishing the Aominaga River on Hokkaido (北海道 相沼内川). You can see in the beginning of the video that Katayama-san has been using the same kebari for quite a few years because you can see the exact same one in his newer Daiwa videos and occasionally on his website.
From what I understand, Katayama-san is using a fluorocarbon level line and Sebata-san is using a tapered nylon line.