I designed these parts to use on a custom stainless steel racks. They can be inserted into a piece of .375"x.028" tubing (cut square) and lap welded. I generally would then file and sand the result smooth. This assembly technique offers a key feature: If a strut is just slightly too long, the builder can shorten it (by rolling it against a benchtop disc sander) quickly and in small increments. In addition, the entire assembly is self aligning - the rack end bolts to the boss and slips into the tube, which can have any orientation necessary. This method is much easier than using tabs & slots, which take a lot of work to align just right and are difficult to adjust the length of.

There's more info & background on these parts, how they're used, and my design & procurement process on my blog. There's also a dimensioned PDF here, and a detailed description of the procurement process and my cost structure here.