This finder accompanied a Sinaron HR 28mm f4.5 and was necessary when that lens was mounted on the now defunct Sinar M body. I've since had this lens re-mounted to Cambo and the finder is surplus. (Rodenstock made these lenses)

The mask has a Sinar part number 493.554.550.
Due to manufacture date of this finder the mask may correspond to a sensor size of 48mm x 36mm. A Google search of the Sinar part number yields no information so someone with an in-depth knowledge of the Sinar back catalogue may know more.

$AU 500 or $USD350
Free shipping from me in Australia

Click image for larger version. 

Name:	sinar28mmfinder04.jpg 
Views:	2 
Size:	213.3 KB 
ID:	143980Click image for larger version. 

Name:	sinar28mmfinder02.jpg 
Views:	2 
Size:	245.7 KB 
ID:	143981