Greetings again.
Well..................not sure what to say now.
Very obviously a one piece item and unusual, to say the least.
Perhaps Keith may be able to assist as to whether or not his brooched item is a similar/same construction.
What I will add though, is that the construction, flat reverse and lack of detail thereon, is very reminiscent to many Sweetheart brooches; however I'm not saying that this a falls into that category.
Apologies that I can't add anything else to assist at the moment.
Regards.
Brian