Data source is the files of Stella M. Mumma.
Data soure is "Early Dauphin Co. Families and their Descendants"
The Dauphine Co. book lists his birth as 18 Mar 1890. I believe that Stella would have known her brother's birth, so the1884 is the one used.
The 1900 Census lists Stella as living at home and being born Feb 1882 and Gilbert as 1884. Mary is listed as giving birth to 3 children, 2 are still alive which would be Stella and Gilbert. There is a conflict between Gilbert's birth with the census and his tombstone.
1920 PA Census, Dauphin Co., Lower Paxton Twp, ED 116, sheet 13 A, 21 January 1920
Mumma Isaac, head, 60, M, PA, PA, PA, retired farmer
Mumma, Mary, wife, 60, PA, M PA, PA
Mumma, Stella, daughter, 33, S, PA, PA, PA bookeeper
Mumma, Gilbert H, son, 31, PA, S, PA, PA, PA
1930 PA census, Dauphin Co., Lower Paxton Twp., ED 22-71, Sheet 7B, April 12, 1930
Mumma, Stella M., head, $10,000 real estate, 40, S, Stenographer, state public service
Mumma, Mary, mother, 68, widow,
Mumma, Gilbert, brother, 38, S, no employment
Shoop's Church Cemetery - West Hanover Twp., Dauphin County, Pennsylvania
http://www.interment.net/data/us/pa/dauphin/shoops.htm
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MUMMA, Isaiah, b. 3 Feb 1859, d. 14 Jan 1927, sp. Mary SHELLAHAMER, [SH]
MUMMA, Mary SHELLAHAMER, b. 6 Mar 1859, d. 27 Jan 1950, sp. Isaiah MUMMA, [SH]
MUMMA, Stella Mary MUMMA, d. 01 Nov 1974, par. (Isaiah & Mary SHELLAHAMER MUMMA, [SH]
MUMMA, Gilbert Henry, b. 18 Apr 1890, d. 11 May 1932, par. (Isaiah & Mary SHELLAHAMER MUMMA, [SH]