According to church records from 1921 and again in 1980, his name is Davis, (was shown as David). Aged 78 years & 8 months
He was known as Davis on the census and in family letters. It may say David on the headstone because his grandfather's name was David Davis. It is very common in the family to use family names. Everyone was named for a relative. His name may have been David Davis Adams or Davis David Adams.
Another bit of info from SHARON NEIMAN KLEINSTUBER (#47125926)
Burial Records of Rev. Jesse Knipe, # 638:
"Davis Adams, born Janry 20th 1780, died Sept 19th & bur'd 21st 1858; aged 78 yr 8 mo."