The designation B-29L was originally assigned to B-29s that were to be
converted as receiver aircraft for the British-developed hose-based
inflight refuelling system. However, the B-29 receiver aircraft were
designated B-29MR instead.
Warbird History--B-29 Superfortress, Chester Marshall, Motorbooks
The Boeing B-29 Superfortress, Mich Mayborn, Aircraft in Profile,
B-29 Superfortress, John Pimlott, Gallery Books, 1980.
Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press,
United States Military Aircraft Since 1909, Gordon Swanborough and
Peter M. Bowers, Smithsonian Institution Press, 1989.