They are out there - rare, but out there. The M-F plane is a redesign of Stanley's model, with, I learned when I had one, different everything - exact fence shape, threads in the threaded parts, etc. Let's see pictures!
The duplex rabbet plane is really quite a handy plane, and under-respected. My main example is a Stanley, but I've also got a Sargent of the curvaceous period (what some folks refer to as the "wave" planes), for no particular reason except that it's purty as can be. This image is from the Sargent plane page:
You can't see the ornate decorative work on the other side of the casting from the fence side. The image below gives a little sense of what it looks like, but it's prettier from a full side view:
