Roman,
The freewheel first gear operates as Chris describes and was fitted to all standard Bristol Saloons from 400 - 406 , it is quite nice once you understand the car and get used to it .
From the photo you attached , if that is your car , it is
Chassis 400/1/669
Car 400/A/404 (the 304th of 421 made)
Engine 85A/1453 (with 3 SU carbs)
Original Colour Metallic Green
Despatched ex BAC Filton on 17/4/50 and arrived in Italy for the Turin Motor Show with another 400 chassis 594
It was fitted with a KPH speedo , RHD and opening rear window .
Currently with Italian registration number MI 157081
Looking at the photos I have of this car the two wires to the Solenoid on the front fuel bowl are not connected .
Pls confirm whether this is your 400
Regards
Geoff
geoff.dowdle@gmail.com