With respect FourWinds, not sure that your postcard is the Dyffryn Ardudwy east tomb.

A pic from of the east tomb from Alvin Coburn's collection from the 1930s (?) looks like my pics from last September:
http://www.geh.org/ar/strip27/htmlsrc/m197927210001_ful.html

Also your dolmen is qute elegant whereas the east tomb is er... a bit ugly. It is obviously not the cuddly western tomb.

Alvin has another photo which looks like your dolmen from the other side:
http://www.geh.org/ar/strip27/htmlsrc/m197927210004_ful.html

If not the east tomb then where can it be? Carnedd Hengwym (south) before it collasped? Any other ideas anyone?

BTW, more of Alvin's great pics are here:
http://www.geh.org/ar/strip27/htmlsrc/coburn_sld00039.html