Need help with sk call sign

Hi thanks for taking time to read this.
My grandfather was a ham operator ,he has been deceased since 1967.I was only ten then and do not know what his call sign was. The only thing that I remember is walking into his shack and looking around at all the lights and radios(ie i think there were radios.) I would like to find out what his call sign was, so that I could see if it is available for me to use.
His name was
Robert A. Martin
Sabattus Maine 04280

Thats all the info that I have. I hope someone can help me out .

Thank You
73's