Martin , around the mid 1920s one particular make used this distinctive exact shaped fuel tank with the right in the middle centre fold along the side, that make being "Vindec" a product of the huge important accessory firm , Brown Bros., LTD 22-34 , Great Eastern Street, London, E.C. The Vindex of those years used a small CC JAP Sidevalve of 292cc and a Sturmey Archer Gearbox the 1923/4 Model as in the photograph being chain cum belt drive and did not have drum brakes also the Front Forks were of Druid Patent and not like the set fitted in your photograph however later models may have had the more undated designs. Max
