Just double checking here - are you sure it is BNC-20BC and not BNC-20CB?
I know of no BNC-20BC. It doesn't appear in either edition of John Brohm's extensive Estes nose cone/kit references. It's not in Carl McLawhorn's reference list on the old Semroc site either.
But the BNC-20CB part I know, and was used in the Estes Nike Ajax kit. I have a surviving model, and wanted to clone it. The version of the BNC-20CB that (the old) Semroc sold me is not a good copy of the BNC-20CB on my original model. Was going to get around to showing the difference to Randy someday.
PS - Erockets already has a very large selection of Estes/Centuri/FSI/Semroc parts, and are actively milling them on their machine. I saw the inventory myself, and it is a huge and extensive selection that is growing.