From Worldconnect Post-em:
Just FYI: James M. Manwaring first married [or at least had a son by] Dora E. Colvin [b. 13 Mar 1887, Springfield, Oakland Co., MI].

Source: South Dakota Department of Health Birth Record Search Site [Records With Birth Dates Over 100 Years]:

Birth Information for MILLARD MANWARING
(State File Number: 691006)
File Date: 04/30/1946
Sex: Male
Birth Date: 07/10/1906
City/County: LAWRENCE
Mother's Name: DORA COLVIN
Father's Name: JAMES MANWARING

http://apps.sd.gov/applications/PH14Over100BirthRec/index.asp
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Dora apparently died soon before the 1910 census was taken as young Millard was "farmed out" with apparent strangers in South Dakota. Her parents, Daniel E. and Margaret [Hayes] Colvin had moved to Covington Precinct, King Co., WA. James M. Manwaring was living in King County [Seattle] too.

Sources:

1910 Census, Reeds School District #16, Lawrence Co., SD, E.D. #41, Sheet 12B, Dwelling #191, Family #190, enumerated 12? May 1910:

Smith, Rhoda, Head, 58, Wd, b. Illinois, f/b Indiana, m/b Pennsylvania
--- Pearl, Daughter, 16, b. Wyoming, f/b Illinois, m/b Illinnois
Manwarring, Millard, Boarder, 3, b. South Dakota, f/b US, m/b US
____________________

1910 Census, 1st Ward, Seattle, King Co., WA, E.D. #61, Sheet 26B; Enumeration District: 61

Mannwaring, James M, Roomer, 33, b. Missouri, f/b Missouri, m/b Missouri, Brakeman Railroad
___________________

In 1918 James Millard Manwaring registered for the WWI Draft in Seattle. He gave his DOB as 28 Dec 1875 and identified "Laura Manwaring [wife]" as his nearest relative.

By 1920, his son, Millard, also had come to Washington where he was enumerated with his widowed grandmother.

1920 Census, Puyallup, Pierce Co., WA, E.D. #224, Sheet 2A

Colvin, Margaret C, Head, 57, Wd, b. Michigan, f/b Scotland, m/b Switzerland
Mannwering [sic], Millard, Grandson, 13, b. South Dakota, f/b South Dakota, m/b Michigan
Colvin, Elizabeth E, Daughter, 23, Div, b. Michigan, f/b Michigan, m/b Michigan

• Married: ABT 1911 ¹

1. Title: GEDCOM File : Counts, Cuntz, Stover Manwaring-1202_import.ged
   Author: Beryl G. Morrill
   Abbrev: GEDCOM File : Counts, Cuntz, Stover Manwaring-1202_import.ged
   Date: 18 DEC 2002