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A simple tube mount for 46mm dia projection lenses to L39, Leica thread mount.
Most "46mm" dia projection lenses are between 46.3 and 46.7mm diameter. Very few are actually 46mm or more, though I have encountered one that was 46.1mm (and too tight for this print).
The tube is 50mm long, and will hold most lenses, 40mm to 70mm focal length (maybe some 80mm). Certainly 100mm and longer lenses require a longer barrel, though you can use L39 extension tubes to achive this.
The inside the barrel has a 20mm wide recess (0.25mm deep, 3mm from the front), this is to accomodate some material to grip the lens and provide a light-tight fit. If the lens is loose enough, the non hook side of sticky velcro works very well (it's between 1/2 and 1mm thick) and will accept several lens of slightly different width. otherwise a 20mm wide piece of card will suffice.
(These projection lenses are almost always petzval designs with fairly large coverage, greater than full frame 35mm).