Thanks Peter, I was hoping you might pick up on this thread.

A link to the stamped D is below:
IMG00056-20100911-1619 | Flickr - Photo Sharing!

Is this consistent to a Draw Lapped Barrell as suggested?

Other pictures of the rifle (For reference) are below:

IMG00069-20100912-1204 | Flickr - Photo Sharing!
IMG00070-20100912-1204 | Flickr - Photo Sharing!

IMG00072-20100912-1204 | Flickr - Photo Sharing!
IMG00073-20100912-1205 | Flickr - Photo Sharing!

IMG00068-20100911-1622 | Flickr - Photo Sharing!
IMG00067-20100911-1622 | Flickr - Photo Sharing!

I apologise about the poor quality of phots!

Thanks everyone, your expertise is much appreciated!