Hey gang, so pleased to have won this on eBay today ! Extremely rare to find these new ! 
This is the carrying case and battery pack for the Sony D-50 or D-5 cd walkman.

Quite the catch, amazing addition to your D-5/D50.  Boxed wow!  There were Three power sources for the D50/D5.  A slide on (rail goes under and clicks in A/C mains #AC-D50 ( for USA/CAN)

EPB-300 ( 6 C Batteries heavy like boat anchor)

And the slide in case you have for shoulder strap ( pretty useless as no skip protection

on the D-50 / D-5.  But beautiful and you can carry it to the picnic then play it