The Steel Tech sets were made by Gilbert and sold by J C Penney

This is a 1929 #4 set

The manual claims Steel-Tech is The Duplex Standard Construction Toy. Duplex refers to the fact that the plates and girders have two sizes of holes, visible in the model below.

Here is a Steel-Tech model on the left and a similar model made with an older Erector. As Erector evolved the newest ideas were used on the main Erector Line, and older forms were used on special sets like the Steel-Tech sets. The same crane in the newer Erector main line was much improved. Notice the two sizes of holes on the top girders of the beam on the left.