If your grandfather was THE Ray Miller, of 1920s Chicago bandleading fame, this is very good news. As far as I knew, RM disappeared without a trace sometime around 1930, the rumor being that he ran afoul of the Mob. I would like to think he survived into the '70s. Of course that would mean he escaped all interviewers, including Phil Evans, who would have loved to ask him about Bix.
There is an excellent Ray Miller CD on Timeless, detailing his life and work as we know it, and that of some of his famous sidemen.
Does Grandpa's trunk offer any further clues one way or another? Any letters or recollections? That 78 rpm record is a start, but it might prove only that gramps was amused to have a famous musical namesake.