The very early production MkI Bren light machine guns were made with two dovetail brackets on the left side of the receiver. The rear one was for the standard rear sight, and the front one was to accommodate two types of optical sights. A mounting for the No.32 telescopic sight (the same one used on the No4(T) sniper rifle) was planned, but never produced. What was made in small numbers by the Plessey company was a "fixed line sight". This was really more like a surveyor's tool than a traditional sight, and it used the same optical element as the Vickers dial sight that was introduced alongside it in 1939. The purpose was to allow a tripod-mounted Bren to be set up with specific limits to its field of fire, and then for the gun to be removed, used on the bipod elsewhere, and returned to the tripod and confirm the field of fire, especially in the dark. This is a somewhat technically complex task, but not one that was actually needed very much for the Bren. As a result, production and use of the fixed line sights was very limited, and the sights are extremely rare today. The mounting bracket on the Bren receiver was rather quickly dropped from production as an unnecessary waste of machining time.