The microphone could be mounted in the front of the helmet or a contact throat-microphone could be used. At first it was only possible for the diver to talk to the surface telephonist, but later double telephone systems were introduced which allowed two-divers to speak directly to each other, while being monitored by the attendant. Diver telephones were manufactured by Siebe-Gorman, Heinke, Rene Piel, Morse, Eriksson, and Draeger among others. Two basic systems of attaching the helmet to the suit were in common use: In one style the perimeter of the corselet was clamped to a rubber gasket by up to 12 bolts, using brass brails to distribute the load and provide a reasonably even clamping pressure to make the watertight seal. In this style the bonnet to corselet seal was independent of the seal to the suit, and often used an interrupted thread system, which involved about a 45 degree rotation to engage the thread fully. The other type used a rubber flange which fitted over the neck hole of the corselet, and over which the bonnet was clamped, usually with two or three bolts. It was also fairly common to clamp the suit to the corselet edge by brails, and connect the helmet to the corselet by two three or four bolts, which could either be studs tapped into the corselet flange, or fold-away bolts hinged to the corselet, and engaged with slots in the helmet flange. Three bolt equipment, (Tryokhboltovoye snaryazheniye, Russian:Трехболтовое снаряжение, Russian:трехболтовка) consists of an air-hose supplied copper helmet that is fastened to a corselet and waterproof suit by three bolts which clamp the rubber neck flange of the suit between the metal flanges of the bonnet and the corselet, making a watertight seal between the helmet and suit., two lead weights attached to the chest and back, heavy boots made of copper and lead, and a diver's knife. Three bolt equipment was used by the Russian Navy in the 19th and 20th centuries. Three-bolt equipment was also made in France by Denayrouze-Rouquayrol from 1874 or earlier, and in Germany by Draegerwerk from about 1912.