Meant to say ...

but I trust labels on old photos, so I'll carry on looking! With a bit of spacial twisting this image http://www.themodernantiquarian.com/post/32951 does look like the image you linked to is Dyffryn.

Looking at several other pictures and removing the modern bricks the orthostats match your picture.

The cairn rubble to the right of the orthostat in your picture matches the rubble in the postcard. So does the wall that can be seen to the left of your old photo.

I would say they are the same tomb, but from different angles (obviously). The postcard is taken from about 120 degrees to the left of where the old photo is taken from.