12th June 2011 I heard from Tony Dellarocca the genealogist and historian for the 14th NYSM Co.E Living History Association. He says:

 'You have a David B. Cumpston listed on your website. I am absolutely sure this is the David B. Compston from our regiment. He was a member of the company of Engineers in the regiment. The engineers were disbanded after the first battle of Bull Run, as there was no authority for an infantry regiment to have a company of engineers. I have cross checked my records, and am quite sure these two men are one in the same. I do have the NYS abstract for David B. Compston, and it jives with the info I do have on David B. Cumpston. David died on February 23, 1863, at Washington DC. His wife's name was Ann, and he had one son I know of, Samuel P. born in 1846. If you have any other info I would appreciate if you would send it along'

I have an entry for David on the American Civil War.  It is great when people take a tiny bit of data from my 400 pages and take the time to make contact. Thanks Tony!
HERE IS MY FURTHER DATA

New York, Civil War Muster Roll Abstracts, 1861-1900

Name Birth Year Birth Place Enlistment Year Regiment
David B Compston abt 1823 Auburn, New York 1861 84th Infantry

Name: David B Compston Age: 38 Birth Year: abt 1823 Birth Place: Auburn, New York
Enlistment Year: 18th April 861 Enlistment Location: Brooklyn for3 years

Muster Year: 23 May 1861 grade Private

Separation Details: Muster Out of Service Separation Date (left the service):
28 Aug 1861 Private at Arlington VA Co. being in excess of Regth, Orig.

Remarks at the bottom of the page say 'Auburn NY 38 Merchant'

Source Citation: New York State Archives, Cultural Education Center, Albany, New York; New York Civil War Muster Roll Abstracts, 1861-1900;Archive Collection #:13775-83;Box #:352;Roll #:1225-1226.

US Civil War Soldier Record Name: David B Compston Age at enlistment: 38
Enlistment Date: 18 Apr 1861 Rank at enlistment: Private
Enlistment Place: Brooklyn, NY State Served: New York Survived the War?: Yes
Service Record: Enlisted in Company Engineers, New York 84th Infantry Regiment on 23 May 1861. Mustered out on 28 Aug 1861 at Arlington, VA.
 
Birth Date: abt 1823
Sources: New York: Report of the Adjutant-General. SEE PHOTOGRAPH
 

1860 United States Federal Census 9th June

Name: Samuel N Compston Age in 1860: 13 Birth Year: abt 1847 New York
Home in 1860: Brooklyn Ward 10 District 3, Kings, New York Gender: Male
Post Office: Brooklyn

Household Members: Name Age
David P Compston 36 value of personal estate $300
Annie Compston 35
Samuel N Compston 13

MY NOTE ON THIS DATA:
I HAVE VIEWED THE ORIGINAL CENSUS WHICH DOES SHOW DAVID'S MIDDLE INITIAL AS P AND NOT B
IT ALSO SHOWS SAMUEL AS N AND NOT P, ALTHOUGH DATES DO MATCH THIS FAMILY.

Source Citation: Year: 1860; Census Place: Brooklyn Ward 10 District 3, Kings, New York; Roll: M653_769; Page: 474; Image: 473; Family History Library Film: 803769.

Tony also provided the following information:
Town Officers from Erin Township in Steuben County, NY. David was town clerk in 1849. He was also a member of the town Fire Department.

Here is a listing from the United States Postal Guide dated 1853 that shows his middle initial as B. Also his initial is B in the other example sited below.
 
United States Postal Guide 1853
Corning, Steuhen, N. Y David B. Cumpston
Cornish, Sullivan, N. H Isrsel Hall
 
From "Pioneer Days and Later Times in Corning and Vicinity 1789-1920"
 
Fire Wardens— James B. Lower, John Hazelton and David B. Cumpston.

At a meetmg of the Village Board held August 27, 1849, it was "Voted, That the Street Commissioners be instructed to remove logs and stumps, and clear a passage through Second street." Also that the Commissioners " open a passage-way for teams on Market street from Wall street to the old River Road