First... the Bormann underplugs you are referring to are brass, not copper.
Answering your question:
I've put extensive, decades-long study into that question. My answer to it is based on personally examining the field-recovered evidence, including hundreds of sawed-in-half Bormann fuzed shells, and the fuzeless 6, 12, and 24-pounder Confederate Bormann roundshells recovered from the CS Augusta GA Arsenal river-dump site. The answer is, approximately 98% of the time, in Army field-caliber shells, a brass underplug means CS-made, and an iron underplug means US-made. I had to specify Army in that statement because we know the US Navy used "oversized" brass Bormann underplugs in its Heavy Caliber Bormann-fuzed case-shot shells.
I encourage readers to verify my observation for themselves. Examine sawed-in-half Bormann shells whose fuze (or, fuze-hole) can be identified as US, or CS. The CS ones are very nearly always brass and US ones nearly always iron. Of course an exception to the rule will turn up occasionally, which is why I said "approximately 98%."
Additional evidence:
The 2.9"-caliber yankee Bormann-fuzed Read-Parrott shells always have an iron underplug. Check the photos of sawed-in-half ones
Regards,
Pete
