Interesting. If possible, ask to examine the pictures (backs as well) and ask about any others that they may still have. Names may be on them. The name Kemp has been associated with various theatres since about 1900, in various google links I ran across, and there may be an earlier one.

The name "Kemp's" thus may refer to a venue rather than a minstrel or theatre troupe.

Not perhaps pertinent, but an Australian arts magazine has 'Kemp's as part of the title; I think it is a fairly recent journal, though.