I like the Summarit- the optical formula is revised from the Xenon, I seem to remember the 4th element has a different curvature.
The Xenon is optimized for F1.5 from close-up to infinity. A Hex shaped aperture is used to reduce the effects of spherical aberration on focus shift.
The Summarit is a 51.1mm lens, at least the 5 that I took apart were all scribed 51.1. The Summarit is optimized for ~F2 close up, front-focuses at F1.5, and is in focus using F1.5 at infinity. Their way of dealing with focus shift. After all- who is crazy enough to use a lens wide-open and close-up. If you are, just use a shim under the rear groups to move them back slightly. This increases the focal length and reduces the distance between the image plane and lens, optimizes for close-up and wide-open, good enough for infinity. About 0.08mm. A Shim from a J-8 works.
The 50/1 Noctilux and 50/1.1 Nokton are closer to the Ultron design, the rear element of an Ultron design split into two of lesser power. Similar to the Xenon/Summarit compared to the Opic: rear element split into two of lesser power.