Can't tell you what it is, but since you wonder if it is civil war related, or at least from the civil war era, I can tell you that in 40+ years of dealing civil war relics, I've never seen one marked "Pat. Pend." (Patent Pending). I'm fairly certain it means the unidentified object was made sometime after the end of the civil war... and perhaps, much later. In general, the use of "Plain Block" lettering in metal marking means latter-1800s at the earliest (and continues into the 20th Century and the present). Metal-markings from the mid-1800s and earlier very nearly always used "Serifed" lettering. (See the illustration below.) A prime example of that is civil war era metal button maker's-markings ("backmarks"). Of course, there's always an exception or two to the rule... but I'm quite certain the unidentified object marked Pat.Pend. is from long after the civil war.
I think the legal option to mark your invention or product with a Patent Pending warning did not come into existence until a few decades after the civil war ended.
