According to R.H. :
A quick magnet test on the screws themselves could settle that. Anything which would rust to the color of those threads would be ferrous, and thus would respond to magnets.
Perhaps while staples were affixed to hold them in place, and the fixture was moved down the beam (or whatever) for the next row of staples? (Though the opening seems a bit small for a normal beam.)
Enjoy, DoN.