What a fantastic sporran! A true work of art.

I agree that those cones/bells are amazing, the openwork with the leather showing through.

It looks like that sporran was originally worn with a leather sporran strap, not a chain strap. Oftentimes old sporran cantles have a pair of metal rings on the back where a chainstrap can be attached in addition to the leather loops for a plain leather sporran strap.

You might be able to see them here, on the back of the Celtic Dragon cantle that's mounted on that sporran:





(The bits of tape are to protect the kilt from the nail ends, as this cantle is held on with two tiny nails, pushed through then bent over, very common on old sporrans.)

Now that thing you call a "D", I had heard called an adapter, just a short bit of leather with two D-rings. They're to allow a sporran with loops for a plain leather sporran strap to be worn with a chain strap.

I've made these myself, they're easy to make with a bit of leather, two D-rings, and two rivets. The modern adapters are much narrower than would be best for your antique sporran, I think.