Hello, If you are still looking for the correct wartime pattern D rings that fit the observer pack I have originals of these and I have had reproduction steel rules made to secure them to the pack. They are £30 a set plus postage at cost.
For those that are unaware or have never seen an original ring they are more of a round cornered rectangle with 2 central bars about 3/16ths of an inch apart. This is where the steel rule slides between. One half is visible on the outside of the pack, hence D ring, the other is inside the pack and accepts all the rigging lines form the canopy. Also, both rings are lashed together on the inside of the pack with paracord so in the event of a badly fitted parachute in a bail out, all the rigging lines would technically be secured to the harness by one hook. Sorry about that - I just caught sight of my anorak hanging on the door... All the best, Stephen