Bill Monroe is a legend of american music. As the inventor of bluegrass music, he is certainly one of the most influential artists of the 20th century. The picture on the left is an early take, probably taken in the mid or late 1940s. It shows also a woman with an accordion, but I can't say who this gal is. Monroe used sometimes an accordion in his recordings, for example in one version of "Kentucky Waltz" and in the instrumental "Bluegrass Special".
Here are some of Monroe's tunes he made in the 1940s for RCA Victor and for Columbia: