Larry;
I'm feeling a little on the lazy side & willing to let someone else do my work. Do you recall off-hand without me having to dig out my proof books, what shot charge the 3" shell was proofed for. As I recall you are entirely correct the 3" shell was proofed for a 1oz load but just don't remember the 3" one. I don't think they were near as common as the 3" length anyway.
My comments were aimed soley at the possibilities of the LC mark. In all probability you were 100% correct in your identification of the chamber in question.
