Notes:
I suspect that Peter was first married to Elizabeth and they divorced not long after the 1880 census. In the 1900 census, Elizabeth lists herself as a widow, but not an uncommon thing for a divorced woman to state. In the 1900 census, Peter Mummaw lists himself as having been married twice. Their birth dates are almost identical. Peter Mumma is last seen in Columbia Twp in 1880 and Peter Mummaw appears in Columbia township in 1900. This is pure speculation without further evidence.
1860 Census, PA, Lancaster Co., West Hempfield, p. 973, 22 June 1860
Frederick Mumma, 36, laborer, b. PA
Harriet " , 36, b. PA
Frederick " , 11, b. PA
Harriett " , 9, b. PA
Peter " , 7, b. PA <=======
The following information is speculative, but is a reasonable fit. If you information that shows this data is in error, please contact Doug Mumma, doug@mumma.org.
1870 Census, PA, Lancaster Co., West Hempfield, p. 475, 14 July 1870
Bucher, Christian, 27, Blacksmith, $2100 real estate, $400 personal property, b. PA
" , Martha, 27, keeping house, b. PA
" , Elizabeth, 2, b. PA
" , Mary, 1, b. PA
Meckley, William, 21, apprenticed to blacksmith, b. PA
Mummah, Peter, 17, apprenticed to blacksmith, b. PA
Johns, Elizabeth, 16, domestic servant, b. PA
1880 Census, PA, Lancaster Co., Columbia Twp., ED 129, p. 151
Mumaw, Peter, 25, brakeman on RR, b. PA/PA/PA
" , Elizabeth, 24, wife, keeping house, b. PA/PA/PA
" , Harriet, 6, dau, at home, b. PA
" , Mary, 2, dau, at home, b. PA
1900: Columbia, Lancaster, Pennsylvania
Peter Mummaw, head, Dec 1852, 49, m: 15 yrs. b: PA/PA/PA, iron worker
Mary E Mummaw, wife, Nov 1865, 35, m: 15 yrs, 2 children, 2 living
James B Mummaw, son, 14, single, lace worker
Henriette H Mummaw, dau, 13, single, lace worker
Adolph Geller, boarder, 19
Harry Ernst, boarder, 24
1900 Census, PA, Lancaster Co., West Hempfield, , ED 121, p. 291
Mumma, Lizzie H., Head, Jan, 1856, 45, widow, 3 children 2 living, b. PA/PA/PA day laborer
Elsberger, George, head, July 1874, 26, m: 0 yrs, b. PA/GermanyGermany, Iron miner
" , Harriet, wife, Feb 1874, 26, m: 0 yrs., 1 child, 1 living, b. PA
" , Vernon, son, Jan 1900, 4/12, b. PA
Mumma, Mary, sister-in-law, Jan, 1877, single, 1 child, 1 living, b. PA
Gable, Harry, nephew, Dec 1897, 2, b. PA
1910 Census PA, Lancaster Co., Columbia Borough, ED 16, p. 81, 212 Perry Street
Mummaw, James B., Head, 24, m1 5 years, b. PA/PA/PA, printer - newspaper
" , Annie M., wife, 25, m1 5 yrs, 4 children, 3 living, b. PA/PA/PA
" , Mary B, dau, 4, single, b. PA
" , Anna, dau, 2, single, b. PA
" , Samuel, son, 1, single, b. PA
" , Peter, father, 57, m2 25 yrs, b. PA/PA/PA, laborer odd jobs
1920 Census PA, Lancaster Co., Lancaster City, 8th ward, p. 132, 36 1/2 Laurel St.
Mummaw, James B., Head, 34, b. PA/PA/PA, ? newspaper
" , Annie, wife, 34, b. PA/PA/PA
" , Mary, dau, 14, single, b. PA
" , Anna, dau, 12, single, b. PA
" , Samuel, son, 10, single, b. PA
" , Henretta, dau, 9, single, b. PA
" , Sarah, dau, 8, single, b. PA
" , Grace, dau, 6, single, b. PA
" , Clara, dau, 3 8/12, single, b. PA
" , James, son, 1 11/12, single, b. PA
" , Arthur F, son, 1/12, single, b. PA
" , Peter, father, 67, m. , b. PA/PA/PA no occupation
I suspect that Peter was first married to Elizabeth and they divorced not long after the 1880 census. In the 1900 census, Elizabeth lists herself as a widow, but not an uncommon thing to do for a divorced woman. In the 1910 census, Peter Mummaw lists himself as having been married twice. The birth dates are almost identical in 1852-1853.