Do you have the death information on Rufus SPRINGSTON? He married Iowa's sister, Bertha CRYSTAL; maybe if you sent for his death cert/copy it might list his mother's name.

Source Citation: Year: 1900; Census Place: Township 1(Middletown?), Lake,California; Roll: T623 88; Page: 11A; Enumeration District: 42.; June 27th,
lines 17-18, 238/239,(same page as bro/William and Iowa)
Name: Rufus SPRINGSTON
Home in 1900: Township 1, Lake, California
Age: 42
Estimated Birth Year: abt 1858
Birthplace: California
Relationship to head-of-house: Head
Spouse's Name: Bertha
Race: White
Occupation: View Image
Neighbors: View others on page
Household Members: Name Age
Rufus SPRINGSTON 42y, h/h, w, m, b.Dec.1857, M=5y, CA -- --, day laborer,owns home free,
Bertha SPRINGSTON 23y, wife, w, fe, b.June 1876, M=5y, 0births, CA IN OH

re: CRYSTAL:
Source Citation: Year: 1880; Census Place: Santa Rosa, Sonoma, California;
Roll: T9_84; Family History Film: 1254084; Page: 135.2000; Enumeration
District: 125; Image: 0273.
Name: Bertha Crystal
Home in 1880: Santa Rosa, Sonoma, California
Age: 3
Estimated Birth Year: abt 1877
Birthplace: California
Relation to Head of Household: Daughter
Father's Name: Benjamin F.
Father's birthplace: Indiana
Mother's Name: Elissia M.
Mother's birthplace: Ohio
Neighbors: View others on page
Marital Status: Single
Race: White
Gender: Female
Household Members: Name Age
Benjamin F. Crystal 45
Elissia M. Crystal 32
Luvina M. Crystal 14
Iowo Crystal 7
Bertha Crystal 3
Franklin B. Crystal 9M

Source Citation: Year: 1920;Census Place: Middletown, Lake, California;
Roll: T625_101; Page: 5B; Enumeration District: 76; Image: 575.
Name: Rufus SPRINGSTON[Rufus Spanngshie]
Home in 1920: Middletown, Lake, California
Age: 62 years
Estimated Birth Year: abt 1858
Birthplace: New York
Relation to Head of House: Head
Spouse's Name: Bertha
Father's Birth Place: Pennsylvania
Mother's Birth Place: Ohio
Marital Status: Married
Race: White
Sex: Male
Home owned: Own
Able to read: Yes
Able to Write: Yes
Image: 575
Neighbors: View others on page
Household Members: Name Age
Rufus SPRINGSTON 62 NY(?) PA OH
Bertha SPRINGSTON 42 CA IN OH
Donald SPRINGSTON 18 son
Jessie R SPRINGSTON 11 dau
Sherrol B SPRINGSTON 8 son

CADI:
Springston, Rufus, sp/B, 65y, co#17/Lake Co, d.2/19/23

Sorry, I've been unable to find William's parents listed anywhere I have searched. Or Nancy's. Lake County is probably the best place to search, maybe some marriage records for the daughters that would provide last names to search for death dates.