The Orion 28mm f6 is an ultra-wide angle Rangefinder lens, which is when the glass is much closer to the sensor than normal, allowing for the lens to have extra compensation for distortion.
This lens can only be mounted to cameras with shorter flange distances than EF. I have adapted it to Sony E for the A7s ii. It could also be adapted to m4/3, although the Russar 20mm and Jupiter 35mm can’t be adapted to m4/3 because the small size of the sensor blocks the rear element of the lens from entering/penetrating the gate , allowing the lens to get as close as it needs to be mounted properly.
The difficulty is not being able to use this lens on any EF cameras, but the low distortion and natural flare pattern of the lens makes the look worthwhile. The set covers the extreme telephoto side because it spotlights rangefinders which are wide angle with a telephoto compression.