They were a Piano maker and musical instrument distributor based in Detroit, Michigan, founded in 1879. There were 4 Brothers originally and the firm stayed in family hands until 1955 and went out of business in 1981, (In 1994 a new Grinnell Bros. Piano Co. started up and is still going with Pianos). 

When they were a major distributor they had their own brand of chordophones, including Ukuleles, called Wolverine. Some of the Ukuleles sold under this branding were made by Martin, (as were some of the Guitars and Mandolins), and the sale of them went on into the 1930's. I have read, but not seen that the branding was also used on Leonardo Nunes made Ukulele too.