I think he is referring to the bearing blocks used to the guide the inner and outer tubes in the arm.
https://fbcdn-sphotos-c-a.akamaihd.n..._5274788_o.jpg
This is the bearing block system from 604's first arm in 2011, which was 3" square outside tubing and 1.5" square inside tubing. The left block inside the top of the outer tubing and the right block inside the bottom of the inside tubing. The bearing system worked great, but even with the large block split into four pieces, there was a lot of machining in those blocks. We redid the arm with much smaller tubing as an off-season project later with delrin slides. It was far lighter, but did have a noticeable amount of friction. However, the machine time was next to nothing, even on a benchtop cnc.
Hopefully this is what you are looking for