The inscription 'Bob, London' is written in ink, with Hockney's initials and '64' written in pencil. As he and Bob were in London in 1965, it is possible that Hockney signed and dated the drawing at a later time, and was mistaken about the date. Both works are similar in style, delicately drawn and highlighted with pink crayon.
Nikos Stangos (ed.), David Hockney by David Hockney, London 1976, pp.101-2, 134