The manuscript transcribes as follows:

“Well they call me Jem Bags for want of a betterer.”  Wandering Minstrel. Fred. Robson

Robson appeared in the role of Jem Bags, a bedraggled Cockney street-singer, in The Wandering Minstrel, a one-act farce written by Henry Mayhew in 1834, which was first performed on January 16th of that year at the Royal Fitzroy Theatre, London, but Robson, who was only a boy at that time, wasn’t seen in the part until, I think, 1852, in Dublin, with revivals in many towns until not long before his death in 1864.  Interestingly, the portrait shows him as himself, not in character, and he is (forgetfully, I suppose) misquoting the line from the play, which is: “I answers to the name of Jem Bags for want of a betterer.”

For a full account of the exchange, see the Green Room post here