What was the position of Oliver O. Howard's brigade of Sedgwick's division in the West Woods? Howard's brigade was the third of three brigades that made up Sedgwick's division--the first was Willis Gorman's and the second, Napoleon J.T. Dana's.
According to Ezra Carmen "Howard's line advanced from the East Woods in some disorder. As it approached the Hagerstown Road, the right of the brigade began obliquing to the right, while part of the left wing (which had been halted) was attracted by the contest around the [Dunkard] Church and began to oblique in that direction. Part of the 69th Pennsylvania followed the 106th Pennsylvania, and part overlapped the 72nd Pennsylvania but was soon moved to the right. Under a fire that struck down many, the 72nd reached the road [Hagerstown Pike]--somewhat broken by the rush of the retreating 125th Pennsylvania through it. It was aligned by dressing to the right and then advanced about ten yards into the woods. Its left, which was near the church, could not fire because some of the 34th New York were in front, but the right wing was uncovered and began firing." The colored map at left is the familiar Cope/Carmen Map and faithfully reflects Carmen's narrative.
Here's the problem. Nearly all of the recent accounts of Sedgwick's advance place the left of the including Howard's brigade much further to the north and in the clover field adjoining the West Woods At left is Marion Armstrong's map that shows the 72nd further north and well away from the church. Richard F. Miller puts the 34th and 125th at the Dunkard Church and, like Armstrong, locates Howard's brigade northward. So does John Priest . Luvass' Guide to the Battle of Antietam, appears to place only two units near the church which are unlabeled but can be assumed to be the 34th NY and 125th Pa.  On the web, the ehistory site at Ohio State University places Sedgwick and all of his brigades at the same map location as Armstrong and others.  Ethan Rafuse places the 72nd Pa. according to the Cope/Carman map but does not note the location of the 69th Pa. and instead shows the 7th Michigan on the right of the 72nd. 
So what to make of this? It appears that there are two map models representing very different unit locations. One representation replicates the Cope/Carman map; the other consistently follows what I will call the Armstrong map (even though some maps here precede Armstrong's map). So who is right? Where was Howard's left? Who wants to challenge Carmen? He was there and spent his life chronicling the battle. Nearly all of the histories above agree that the 34th New York angled down to the church--that is not in dispute. What is in dispute are the locations of the 69th and 72nd Pennsylvania of the Philadelphia Brigade. Consulting the ORs do not provide a definitive answer (is there one?). For example, here's Col. James A. Suiter's report for the 34th New York:
"From some cause to me unknown, I had become detached from my brigade, the one hundred and twenty-fifth Regiment Pennsylvania Volunteers being on my right. On my left and rear I was entirely unsupported by infantry or artillery." -- Col. James A. Suiter's (34th New York) OR.
Where are the 72nd and 69th? According to Carmen/Cope, they should have been right behind the brigade. If so, why does Suiter say he was unsupported in the rear by infantry?
Joshua T. Owen's report on the 69th Pennsylvania further confuses things. He writes:
"With some confusion upon the left, the brigade retired. The Sixty-ninth, One hundred and sixth, and Seventy-first Pennsylvania Volunteers retired in good order; the Seventy-second Regiment Pennsylvania Volunteers, however, being on the extreme left, subjected to a heavier fire, and the first to encounter the panic-stricken fugitives from the left, did not retire in the same good order as the other three regiments, nor was it reformed, nor did it rejoin the brigade until a late hour in the afternoon." --Joshua T. Owen, (69th Pennsylvania), OR, 9.21.62.
According to the Carmen/Cope map, the 125th and 34th appears to have retreated through the 69th and 72nd positions but Owen reports that only the 72nd encountered the unknown "panic-stricken fugitives from the left." Could these "fugitives" have been the 7th Michigan and the 42nd New York which were to the left of the 72nd in the Armstrong map?
The 72nd's commander Col. Dewitt Clinton Baxter left no report and the division commander Oliver O. Howard does shed any light on the location of the left of the division.
I hope someone who finds this entry may be able to clear up the two map representations and help find Howard's left. If you are out there and can help, please post.
 Joseph Pierro, ed., The Maryland Campaign of September 1862: Ezra A. Carman's Definitive Study of the Union and Confederate Armies at Antietam ( New York: Routledge, 2008), p. 264.
 Marion V. Armstrong, Jr., Unfurl Those Colors! McClellan, Sumner, and the Second Army Corps in the Antietam Campaign (Tuscaloosa, AL: The University of Alabama Press, 2008), p. 186.
 Richard F. Miller, Harvard's Civil War: A History of the Twentieth Massachusetts Volunteer Infantry (Hanover, NH: University Press of New England, 2005, p. 166).
 John Priest, Antietam: The Soldier's Battle (New York: Oxford University Press), p. 121.
 Jay Luvaas and Harold W. Nelson, eds. Guide to the Battle of Antietam: The Maryland Campaign of 1862 (Lawrence: University Press of Kansas, 1987), p. 166.
 Map of General Sedgwick's assault on the West Woods, retrieved on August 10, 2009 at http://ehistory.osu.edu/uscw/features/regimental/pennsylvania/union/71stPennsylvania/antietam.cfm.
 Ethan Rafuse, Antietam, South Mountain & Harpers Ferry: A Battlefield Guide, (Lincoln: University of Nebraska Press, 2008), p. 71.