American Optical Stereo Microscopes

Eyepiece diameter is the "standard" 23mm.

The "Cycloptic" designation refers to the fact that a single large common objective lens forms a single image which is viewed by the pair of eyepieces. This seems to be the usual arrangement with modern stereo microscopes, but was somewhat unique at the time (1957). The objective lens provides a fixed and generous 4 inch working distance. Truly old stereo microscopes were in reality a pair of independent microscopes mounted side by side, each with its own objective and with a view converging on a common subject.

Also note that the turret that is rotated to obtain different mangifications holds a set of Galilean telescopes (or Galilean lens systems). These get used in both forward and reverse directions for different effect.

Various "turrets" were supplied to allow different magnification choices. No optics in the turret at all yields 15 power (so a user might expect this to be the clearest view, since the fewest optical surfaces are involved).

The "J" turret offers 7, 15, and 25 power.
The "K" turret offers 10, 15, and 20 power.
The "M" turret offers 7, 10, 15, 20, and 25 power.

The above powers are with a 10x eyepiece.

The J and K turrets have 4 positions (two are identical at 15x). The M turret has 6 positions (with two identical at 15x).

The astute reader will have figured out what is happening. A J or K turret has one set of optics, used either forward and reversed, and a clear path (and it doesn't matter which way you look through a hole). The M turret has two lens systems, each of which can be used forward or reverse for different effect, and a clear path.