I looked on a Social Security Death Index website, http://ssdi.rootsweb.ancestry.com/ and I see there is one good possibility, someone named Robert F. Graf (no full middle name is listed), who was born December 28, 1919, and whose Social Security number was issued in Illinois. That man died in California in 2001. I looked on the internet for an obituary for him, but I could not find one.
I also looked online for obituaries for Phillip and Frances Graf, hoping that an obit would mention Robert, but again, I could not find obituaries for either of them.