Please help identify Singer & Sons credenza and if Gio Ponti design  

Bertha Schaefer
The cabinet is a Bertha Schaefer design for Singer. See, e.g.:
http://www.1stdibs.com/furniture_item_detail.php?id=401889
Not that rare, though obviously not common. I dunno anyone who collects B.S. designs, for what it's worth.
Value -- who knows? Maybe a few hundred U.S. dollars on Ebay and a few thousand at a fancy NYC / Paris / LA store?

THANK YOU FOR THE INFORMATION
Thank you so much for the information. I appreciate you sharing your knowledge. I am amazed of all that is to be learned in collecting modern design and this is a fantastic site to learn it.