Sun Umbrella is an item found in Red Dead Revolver.
|| This will keep the hard sun off your downy soft face.
|| —Store description
As with all similar items, the Sun Umbrella is not used in the game beyond unlocking a journal page.
Bob Larson's journal page offers no hints as to how the Sun Umbrella or its store description relate to him.
The Sun Umbrella can be purchased from the General Store in Brimstone after completion of the mission "Railroaded". If not purchased, it is not available during subsequent visits to Brimstone.