Jon - at least you do get to enjoy reading about this one -
The feet are different. The Davis VS underfeed has a noticeable needlebar offset to the right, the rotary has a noticeable offset to the left. The underbraider and shirr plate for the Davis Rotary are the same as that for the Free Rotary. However the feet for the Free Rotary are narrow gap with needle centered, so that part is very easy to tell the difference.
I'm pretty sure this machine predates the Free Rotary, so maybe Free got that style attachment from Davis. I've not seen it used anywhere else. It is an exact fit, so the distance from the needle bar to bed attachment holes is the same, but the needle bar to presser bar distance is different.