The apertures of the CIRCLE medium format mounts have a diameter of 50 mm.
This is a new mount that I created in 2021, inspired by a suggestion Timo Puhakka made on the MF3D list. Unfortunately, I didn’t go back and read that thread before designing this mount, and I didn’t think to make the rear apertures square to aid with rotational alignment while mounting. In my limited experience with these mounts, I haven’t had too much difficulty mounting. If using a dedicated stereo camera, the edge of the film can be aligned along the bottom edge of the mount to help with rotational alignment.
These mounts are a similar design as the cardboard mounts previously sold by Kiewa Valley in Australia and by Rocky Mountain Memories in the USA. These used to be called Alpers or Spicer mounts. The apertures are spaced 62mm apart, making them suitable for viewers with an interocular distance of approximately 65mm (or larger). The overall dimension of the folded mount is 132mm x 80mm. The rear apertures are a bit larger from the front apertures such that the stereo window is always defined by the front apertures, even if there is a slight error in folding. The corners of the front apertures are rounded. The cardstock is heavy weight (110lbs), black in colour, and is advertized as acid and lignin free. The mounts are creased for easy folding, but will be shipped unfolded.