That sounds a bit more realistic than my explanation.It's also important to note that the slide is precision machined from a solid piece of steel bar stock. It is not stamped, molded, extruded or any other such cheap non-sense.
I remember seeing pictures on this forum of a USP slide "in the raw" and in various stages of completion. I have been unable to find these pictures since. If anyone can post them up or have a link I would appreciate it.