The apertures of the BASKIN EXPANDED medium format mounts are 55 mm x 48 mm (width by height).
This particular mount was created by Ted Baskin by intentionally mounting slides in the rear apertures of the Medium Format Wide / Reduced Height mount and expanding the “front” apertures with a knife. This mount was created to save on the labourious process of cutting the apertures by hand. It also has rounded apertures in the front to match the same standard of other mounts, Note that this mount is really quite flimsy due to the thin septum separating the apertures. It works best when inserted into a plastic sleeve to provide some additional rigidity.
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.