I'm the museum registar at Prairie Grove Battlefield State Park in NW Arkansas. This week a guy came in and donated a number of items that his father found here back in the 1940's and 50's while hunting for arrowheads. Amongst the items is what appears to be a Marshall Arsenal shell (looks similar to a Parrott but it's shorter and has a base identical to the one on p.194 of the Dickey and George book). It's about 7.5 inches tall (sabot missing)and just shy of 3 inches in diameter.
The only rifled pieces the Confederates have here that we know of were two rifled 6-pdrs that they had captured at Lone Jack, MO. They also had four 2.25 mountain rifles that had their carriages damaged at Cane Hill and had been sent for repair and this shell is too big for that anyway. The other guns the Rebs had were mountain howitzers, field howitzers and 6-pdrs.
The guys father wrote a statement years ago about finding this stuff at PG, so outside the possibility that he found it somewhere else and had forgotten, any ideas???