The cap, which was found during a clear-out, included a description which said Sgt Slater had lived at 1 Backmoor Crescent in Norton, Sheffield, and attended High Storrs Grammar School.
He was the son of Harold and Ivy Marie Slater and had three siblings called Leslie, Betty and Barbara.
Dr Jones said out of all the siblings, only Betty, 97, was still alive.
She said her uncle Leslie died in the early 1990s while her mother, Barbara, who was 19 years younger than Sgt Slater, died in 2017.
Archive records show, external Sgt Slater was one of a crew of five who were killed after their aircraft was shot down over the Dutch village of Slagharen on 6 October 1942.
