Was the headstamp on the grenaded round "Federal" or "FC"?
Assumption is that these are the only two choices, and that the headstamp portion provided went with the casing stuck in the chamber. A lot of assumptions and speculations that would take a lot of time to establish, prove, disprove in court.
So if it is not going to court, why not accept the S&W offer ? I suspect a more refined approach could be taken with Federal, but I also suspect the ammunition is so old that Federal will not rise and accept responsibility either. However, who knows, other than the Shadow that is ?