Sponsons were built separately from tanks, not necessarily by the company that built the actual tanks.
As far as the Mark I tanks of 1916 are concerned male sponsons, originally the only ones contemplated, came from Sir W. G. Armstrong, Whitworth and Co. Ltd. of Newcastle upon Tyne.
Female sponsons, when it was decided to make them in April 1916, were made by the Metropolitan Carriage, Wagon and Finance Co. in Birmingham.
By the time the first tanks were ready, the sponsons were not, so when the first tanks arrived at the Elveden Training Ground there were no sponsons for them. As a result, early driver training was done in tanks without sponsons.
This practice was criticised by one young officer, Philip Johnson, when he arrived at Elveden. As Johnson pointed out, a tank running without its sponsons was narrower, lighter and much better ventilated than it normally would be, giving the driver a false impression. What is more, when the sponsons ultimately did arrive it was found that they would not fit. They discovered that the hulls of the tanks had ‘worked’ by being driven without sponsons, so none of the bolt holes would line up. New holes had to be drilled around the edge of each sponson before they could be bolted up.
In any case sponsons had to be removed from tanks every time they went anywhere by train. A tank with its sponsons on was too wide for the railway so before they could be driven onto a train the sponsons had to be unbolted and lifted off. Sponsons weighed 1 ton 15 cwts. each and the normal method of lifting them off was to erect a couple of steel trestles on the roof of a tank, with chain lifting purchases at their ends, and then to lower them onto special sponson trolleys that could be towed behind the tank when it climbed onto the train.
The design of the male sponson is attributed to Eustace Tennyson D’Eyncourt, the Director of Naval Construction and chairman of the Landships Committee. The design was based on that used on warships for mounting lesser guns. In the case of the Mark I Tank the gun was a French Hotchkiss design first taken into service by the Royal Navy in 1885. In 1915 it was modified for use in tanks, it was known as the six pounder, single tube, with a barrel length of 40 calibres. It was a 57mm rifled gun described as quick acting, which refers to the action of the breech and the fact that the ammunition, when it was loaded, came as a single item, shell and shell case or cartridge. There was a large door in the back of each sponson which was the normal way of getting in and provided an easy escape route in an emergency.
The female sponson was an ungainly thing, designed in a hurry to fit the same aperture as the male version. It mounted a pair of water-cooled Vickers .303 machine-guns with the gunners sitting on bicycle saddles to fire them.
Their vulnerable barrels were encased in an armoured sleeve so extended fore sights had to be fitted. Each gun swept an arc of over 100 degrees, from the side of the tank out to the beam, but for this reason it was impossible to fit a full size door and the small one fitted instead could be a death trap if the tank caught fire in action.