Most of my feet have the two little prongs, but my buttonhole and rolled hem feet both have bars. Everything I've seen is interchangeable here. I suggest (if it's not too far) to go to your local repair shop/quilt shop (mine is 2 in one) and show them your regular sewing foot & ask what they have.
http://www.pfaffusa.com/2703.html
I have the metal version of this foot
http://www.thecolorfulworldofsewing....sser-foot.aspx