In October last year, there was some speculation as to the reason why bikes, especially track bikes, were built with rearward opening rear dropouts, and I believe that this issue wasn't resolved.
I've just bought a 1939 Claud Butler catalogue, which states that the purpose of these dropouts is to allow a change of gear ratios (i.e., by changing the rear fixed sprocket) without having to alter the chain length each time for a larger or smaller sprocket. The wheel would simply be located forward or rearward in the dropout to adjust the chain tension according to sprocket size.
Neil Foddering
Weymouth, Dorset, England