The "S" designation on the 28S was for "Special Small Motor"....which you can obviously see. And they were designed for getting into tight spaces, especially for behind-the-grille mounting. Federal also offered a couple of "G" model 28s and 76s which were designed for grille mounting on specific-year Chevies and Fords back in the late '50s and early '60s.
I had a 28S several years ago and was pleased with it. They may look strange, but they still work quite well.
You'll also notice the different bracket, the saddle-type. Because these motors aren't like the "starter-type" motors seen on the larger sirens, the bottom of the motor can't be tapped to bolt the bracket to the bottom of the motor like you see in the common 28s, C4s and 76s.
I bought an unusual siren a few years ago off ebay that came from a small F.D. somewhere in MA. The front of the siren was all chrome, and appeared to have originally been a 66G. But it had a B&M saddle bracket. First time "juice" was put to the siren, it turned very slowly. Sent it to my late friend Harold (whom I've mentioned elsewhere on the board) who checked it out. Turns out that the brushes were mounted wrong. That was corrected and it screamed! It also had a slightly better roll than the average 60-series had. We did some checking, and it turns out that the siren was originally a 6-volt siren that had been run too much on 12-volts. The motor was a replacement, and wasn't a siren motor per-se.
