The very first trial frame first came along in the latter part of the 1800s. It consisted of a lower half that could hold a loose lens.
This loose lens was then held in place by a spring-like clip above. Over time this evolved to the ability to hold two lenses on the same side with the cylindric lens being able to be rotated.
This Oculus DRGM trial frame is in the style of about 1930. It was made by the famous German company,Oculus, and is signed “Oculus D.R.G.M.”. It consists of a steel frame with a spring clip in the rear to hold the spherical lens. In the front it has a spring clip to hold the cylinder lens. There is a black disc measured into 360 degrees. Rotation of the cylinder lens would have to be done by grasping the lens and rotating it within the frame.
The PD (interpupillary distance), vertex distance, lens height and temple length are all adjustable.
This trial frame allows for one-sided pupil distance adjustments.