According to Wikepedia 'From the 4th season on, Steed's signature cars were six vintage green 1926–1928Bentley racing or town cars, including Blower Bentleys and Bentley Speed Sixes '. The one he used most however was a 3-litre Vanden Plas open 4-seat tourer with flowing wings. Corgi toys made a die-cast of this, some were the 'proper' green and some were red.
The only blower Bentleys were 4.5 litres, although the Speed Six and Big Six did have a large cylindrical dynamo in the same location as the Blowers' supercharger. This is the one my Dad owned back in the 1970s and people often mistook it for a blower. Utterly fabulous car!: