I am not sure of the introduction date for the 60 degree, 8hp (or 7-9 as it is listed here) Blackburne twin, but the earliest advertisement I have is from The Motor Cycle issue of 19 December 1918. Given the short elapsed period since the end of the war, it seems likely that the twin must have been first produced some years earlier, say 1914 - 1916. The successes listed in the advertisement go back to 1913, but were these for the twin of the single? Can anyone confirm the date?
The motor remained available for a number of years with seemingly little change. Here we see it 4 years later in the 1922 Blackburne publication "Hints and Tips". Labelled as the 5/6hp, those cylinders look more like the 8hp model to me. The 5/6hp (700 cc) model had dimensions 71mm x 88mm while the 8hp (1000 cc) was 85mm x 88mm.