This is what causes the horizontal stringing of shots and/or shots to the left or right.
This paintng "Turkey Match at Sarasota Springs" shows the plank rest when the artist lived in that area sometime before the 1870s when he painted it.
Looking at this something occurs. The "chuck match" common to the area south of the Ohio River is less practical in states that are farther north. Shooting a match in winter with any significant snow on the ground would be impossible using chunk rules. It would be only an inconvenience with the plank rest.
I would also point out that the plank would be very effective as a rest behind a stockade.
All this thinking, as usual, raises more questions.
We do have to remember that rest matches were very common back in the day.
So far as I can tell they were more common than offhand matches.
In Reverend Joseph Doddridge's "Notes on the Settlements and Indian Wars of the Western Parts of Virginia and Pennsylvania, 1763-1783", he is describing a log or stump used as a rest. With pads of moss or other soft material to cushion the rifle.
Stating that "The present mode of shooting offhand was not then in practice. This mode was not considered any trial of the value of the gun, nor, indeed, as much of a test of the skill of a marksmen"
This was found on pg 79 of LaCrosse's "The Frontier Rifleman".
This is just one account but its a pretty good account of a rest match using what ever was available and from reading his account he saw more than one. Stating that "Shooting at marks was a common diversion among the men< when their stock of ammunition would allow it."
Dan