This camera is based on the chassis of the Lewis machine gun and was intended to train pilots in air combat. The Mk 111 H was sold commercially after World War I and remained in use by the RAF until the early years of World War II. The targeting screen has sometimes a grid rather than circles. Some models are designated G-3, these are from a batch sold in the late 1930s. Mk1 and Mk 11 versions were not produced by T-P. Hythe was the location of the Gunnery School.

The mechanism can be divided into three parts, the barrel containing the lens and shutter, the middle section containing the film and film advance mechanism and the rear section containing the release.

The shutter itself (G) operates on a B setting, it is connected to the release mechanism by a wire (g1) and coiled springs. When the release is operated the wire is pulled to open the shutter, a spring closes the shutter.

The camera is set by drawing back a bolt (M) via an external knob (m). This draws back a bar (g) to the right (as depicted in the diagrams below) of the shutter release lever (g2). Drawing back the bolt M also pulls on a bar (k1), at the end of k1 is a chain wrapped around a drum containing a spring (L), this has the effect of tensioning L. The rotation of the chain and the drum moves two gear trains (k), which advance the film. In the drawn-back state the bolt M is held by a catch (c1). When the release trigger (C) is pressed c1 is rocked releasing M which is now pulled by spring L. In its movement the shutter is operated when g rocks the lever g2 which pulls the cord g1. M also operates the frame counter R.

The top of the camera is shaped to take a dummy magazine, fitting the magazine operates a punch which marks the film. The magazine fits on the spindle f, when fitted a plunger (n) moves a lever (N) which operates the punch P.

Immediately in front of the film is a graticule (h) which superimposes targeting circles or a grid on the film. To align the camera sights with the graticule the film holder is removed and replaced by a mirror, this reflects the image of the graticule allowing it to be aligned with the camera sight. With the mirror in place the shutter has to be opened, this is accomplished by inserting a short wire rod into the shutter area and moving the shutter lever.