From Dennis' blog: Bat Byrnes at Bathurst, New South Wales on the Mt. Panorama circuit, end of the 1930s.
-Richard- The bike looks like it started life as a 1935 racing specification machine as the frame has the flat bottom cradle, low front engine lug and lightened frame lugs. The picture dates from no earlier than 1939 as the engine dates from at least this year and it looks like a 500 as it has an 11 fin barrel and the cambox is tucked well up under the tank. The forks are 1936-onwards standard Inter' but fitted with parallel racing check springs and the front brake is a 1937-on conical type. All this is typical of how these bikes were up-dated during their racing lives and can lead to a bike's original specification becoming blurred.