There is no aperture ring on this lens. There is no focus ring either. To focus it, you walk: forward if your subject is too far, back if it's too close, because the glass itself is fixed at f/0.75 and fixed at whatever distance the factory set it to, forever. That is the entire interface. No clicks, no markings, nothing to turn.
That isn't a design choice a photographer would make. It wasn't made by one. The Rayxar E50/0.75 was built by De Oude Delft, a Dutch optical firm founded in Delft in 1939 as Van Leers Optische Industrie, renamed during the German occupation to the more anonymous-sounding name it kept for the rest of the century. The company's real business, the one that paid the bills long after it toyed with rangefinder lenses in the late 1940s, was medical imaging: X-ray equipment, TB-screening cameras, the unglamorous machinery of diagnosis. The Rayxar's job was to sit at the tail end of that chain. An X-ray image intensifier converts a faint radiation image into a glowing phosphor picture, and something has to relay that glow to film or a television tube. That something was this lens. Collectors and sellers describe it plainly: designed for radiography, not photography.
The fixed everything makes sense once you know that. A lens bolted permanently behind a fixed-size intensifier tube has no reason to focus or stop down. It just needs to be fast enough to grab a dim image before the light runs out, and f/0.75 is about as fast as glass gets. One forum regular who'd handled the lens put it plainly: it was developed for x-rays and never meant for photography, and the spherical aberration on offer proves it.
The back focal distance tells the rest of the story. On the original barrel, it's reportedly under a couple of millimeters, so close that the rear element practically kisses the sensor it was designed to feed. There's no room in there for a Leica M mount, or any mount. Getting one onto a camera means surgery: the rear optical group has to come off, exactly as it did on the Camera West example, which lists the removed group as an included extra, like a keepsake from an operation.
What drew photographers to a hospital cast-off in the first place was the number on the barrel. F/0.75 beats nearly anything sold for a camera, and it invites comparison to the one lens everyone already knows about: the Zeiss 50mm f/0.7 that Stanley Kubrick used for the candlelit scenes in Barry Lyndon, of which only ten were ever made and three went to him. That lens is essentially unobtainable. This one, weirdly, showed up on eBay. Bellamy Hunt of Japan Camera Hunter tracked one down around 2014, already modified to Leica M mount, and wrote it up as the practical answer to an impractical dream. PetaPixel picked up the story the same year, framing it as the fast lens ordinary money could actually buy.
So the object you're looking at spent its working life trying to be invisible, a clean conduit for information a doctor needed to see clearly. Repurposed, it does the opposite: glow, blur, and spherical aberration so pronounced it becomes the whole point. The flaw a hospital would have rejected is the reason anyone wants it now.