No, plain matte is just that- a plain matte surface. There's also matte with split image, matte with split image surrounded by a microprism ring, matte with central microprism, plain matte with grid lines, and two with scribed lines for the regular and panoramic 35mm backs: one with split image with microprism ring, and one plain matte.
If you have no luck here, try BUG (Bronica Users Group), or KEH. KEH is very trustworthy as to condition. I also saw used ones on B+H's or Adorama's site a while back.