Lorenzo, the necklace is lovely and though I'm a bit late to the game I'd want to add my thank for allowing us to see it in detail.
I think you're right that apart from the tiara frame and its screws nothing is missing.
The pin on the left side of your photo should be one part of the brooch fitting for the central element. The image is not clear enough to make it out, but I assume the socket of the pin has a carved screw thread in order to screw the socket directly into one of the screw holes on the left and right side in the back of the central element. The pin should be adjusted horizontally. The small pig-tail hook will fit into the opposite screw hole and is meant to secure the pin. It's the most common and quite simple way how brooches were secured in the 19th century. The tip of the pin will probably stand out one or two millimeters to the side in order to make sure it won't slip out.
Regarding the small brooch fitting my guess is that the existing screw on top of the centre element fixes the delicate bow with the round diamond and the attached drop to the outer part of the central element and it's thus possible to detach this part. The small brooch fitting might turn this inner part of the bigger brooch into a small brooch.
For the extra bit of chain it's difficult to make a guess without seeing it, but it might be a replacement for the four diamond swag elements on the left and right side of the central part. The central element (or possibly only the small bow with attached diamond drop) would rather have the chain attached to its sides than being a pendant suspended from a hook.
As for the tiara frame - the frame itself will probably be neither very difficult nor expensive to recreate. The screws might be more of a challenge. I too would contact Bonebakker for the original design and probably for the reproduction as well.
--Previous Message-- : I would like to know the opinion of you : experts in this board. My family owns a : Belle Époque "multi-purpose" : necklace by the Dutch royal jeweler : Bonebakker, that used to be convertible into : broach/pendant or a tiara, but the tiara : frame is lost. Even if there are no : opportunities to wear it as a tiara (apart : from a family wedding, not in sight) I would : like to have it completed. I assume that : going to the original maker or another : "royal" jeweler would cost more : than the value of the piece. Would you go to : any jeweler, or just keep it as it is? :
on March 5, 2014, 1:23 pm, in reply to "