It looks like William bOHc1807 died sometime between 1870 and 1880, possibly in Lake County; Nancy bOHc1820 died betw.1880 and 1900, also possibly Lake County (according to the 1870 & 1880 CA census info). Perhaps someone on the NORCAL or Lake County message boards can help you find a death date; I looked in the pre-1905 indexes but couldn't find their names listed in Lake County.

Source Citation: Year: 1870; Census Place: Zem Zem, Lake, California; Roll: M593_73; Page: 417; Image: 212.; lines 34-40, 132/112,
Name: Nancy Springston
Estimated Birth Year: abt 1820
Age in 1870: 50
Birthplace: Ohio
Home in 1870: Zem Zem, Lake, California
Race: White
Gender: Female
Value of real estate: View Image
Post Office: Zem Zem
Household Members: Name Age
W Springston 63y, m, w, Farmer, $100/estate value, $1000/personal value, OH
Nancy Springston 50y, f, w, keeping house, OH
Rufus Springston 13y, m, w, CA, attd school,
Isabel Springston 7y, f, w, CA, attd school,
George Springston 6y, m, w, CA
William Springston 4y, m, w, CA
Thomas Smith 35y, m, w, Stone cutter, NY

Source Citation: Year: 1880; Census Place: Middletown, Lake, California; Roll: T9_66; Family History Film: 1254066; Page: 32.1000; Enumeration District: 50; Image: 0574.
Name: Nancy Springston
Home in 1880: Middletown, Lake, California
Age: 60
Estimated Birth Year: abt 1820
Birthplace: Ohio
Relation to Head of Household: Self (Head)
Father's birthplace: Connecticut
Mother's birthplace: Connecticut
Neighbors: View others on page
Occupation: Farmer
Marital Status: Widowed
Race: White
Gender: Female
Household Members: Name Age
Nancy Springston 60 Wid OH CT CT
George Springston 19 sngl CA
William Springston 14 sngl CA at school

Also found the SPRINGSTONs in Napa Co in 1860.
Nanc