A nitpicker, he say: either it's a blown 4½ or it's 1929!
The Birkin cars were due to appear in 1929 but were not ready so unsupercharged 4½s and 6½s raced.
I'm not enough of a Bentley nut to know which registration numbers were used, but the photo seems to show a comp. no. 5 - but that was a Stutz in 1929 - and the Blown cars were on "UR" (Hertfordshire) registrations.