If I recall correctly, most 50mm Summicrons unscrew the whole optical block this way. The collapsible doesn't and I'm not sure about the current version with the built-in hood but all the versions in-between do.
The whole optical block of the 35mm pre-asph Summilux comes out this same way as well. It's just difficult to grab the very small area in front of the aperture tabs without adding force to them also. This is very convenient if you have a sticky focusing helicoid and want to get it cleaned. Please don't blame me if you seize up your 35 'Lux focusing helicoid.
The 10.5cm f/2.5 Nikkor LTM front optical block will unscrew this way allowing access to the aperture. Hold the focusing barrel, not aperture ring, and using a piece of rubber to grip the chromed brass front, just twist. The threads are VERY fine pitch and not hard to gall up at all.
Canon 50mm f/1.2 comes apart the same way as the Nikkor above. There *may* be a grub screw holding the front optical block in and if you turn against it, the screw will trash the threads.
Phil Forrest