Extrusion would be my guess also as to how they are made. I suspect the notches cut in the horizontal flat bar to accomodate the spline on the rod bars keep it from rotating so prisoners cant rotate them making a huge amount of noise.
Steel bars are rolled through a forming mill. Softer metals can be extruded: aluminum, brass/bronze, copper. The ridge is to lock the vertical bar into the crossbar, like a key on a rotating shaft.
Jeeze! 35 responses and still no one got it right.
The welded line makes it impossible to use a pipe cutter.