thin steel plate and these replacements were screwed in with the hex in the head being the clue. A good
look though says these plates were made in a foundry that pressed some pretty substantial steel parts, and that indicates they were capable of fabricating the plates however they wanted, and the hex in the heads is not nearly deep enough to use with a hand tool. My puny knowledge of steel fabrication says this job was done in a big machine specially set up with a jig to do this.
Is there any hint of threads in the holes? Or damage from removing the studs? I don't know that this is important because I don't see how you would install any new ones without help.