|If you know when your grandfather died it would help very much. One born March 27, 1926 died Nov. 1982.|
The other possibility is Ernest Rivenbark b. 1919 who lived in Duplin and was the son of Ernest Rivenbark Sr. and Lizzie Maybella Robins who married 11 Jan 1918 in Burgaw, Render, NC. Ernest Sr. is s/o Willie S. Rivenbark and Dollie. Lizzie aka Mabel is d/o Williams and Mary Robbins.
Ernest b. 1919 had Charlie b 1922, Edward b. 1924, Harold b. 1926 and Virginia b. 1928.
If your grandfather was married by 1940, he and Eula should show up when the 1940 census is fully indexed. If you can get the names of your mothers aunts and uncles, and/or your aunts and uncles it will help.
There was a third possibilty, William Robinson Rivenbark had a son Ernest born 1921.
There were a lot of census and birth records, I may have missed something or gotten them confused. Check them at the link below. Also, Ancestry.com now allows guests to sign up for membership w/o entering credit card data and that allows free browsing of the 1940 census so you can browse it now or search it once it is indexed.
P.s. the information I gave is derived from census, birth and marriage records, not anyones family trees.