As the Po’ Ramblin’ Boys emerged as one of the most in-demand and important bluegrass bands of the past 10 years, its founding mandolinist C.J. Lewandowski didn’t focus only on the group’s art and career. C.J., now 38, became an avid collector and caretaker of music history, a board member of the International Bluegrass Music Association, and a record producer. All of those pursuits informed his latest project, a special one for the genre, because Keep On Keepin’ On is the final studio album by bluegrass Hall of Fame singer and mandolin player Bobby Osborne.