The front forks slide into the machined hole of the crossbar. The hole was beveled to get good penetration of the weld. If two pieces of flat steel are butted together and welded the weld might not penetrate all the way through. The idea is to bevel both pieces to create a v-joint (like a valley) then the weld will fill the v-joint and penetrate to the underside. Its a lot easier with flat plate. Pipe or tubing as we call it is a little trickier.....