Painted ground cloth
Someone suggested that calling it an "oil cloth" was confusing folks. It's painted lightweight canvas drill with a linseed oil coating. 74" x 48". Very common with Confederates and civilians, but also present as a private purchase item with Federals to supplement a gum blanket. Same construction as a knapsack or haversack. Reputable vendors charge $65 plus shipping (3-4 lbs.). Asking $50 plus Parcel Post shipping.
Treasurer, The Rowdy Pards
'In the end, it's the history, stupid. If you can't document it, forget about it. And no amount of tomfoolery can explain away anything that makes history (and living historians) look stupid and wrong."