I only had the 28mm Summicron Close focus for a few short weeks during the spring of 2023, Quite long enough to put it through it's paces, but unusually there were no trips to foreign places. 
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This is the third version of the Leica 28mm Summicron. 
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The first version (11604) was released in 2000 before the age of digital, the basic optical design of 9 elements in 6 groups was already established, and like the other two versions it had 10 aperture blades. It also had an immense plastic lens hood, which was rather ugly, and prone to interfere with the rangefinder.
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The second version (11672) was released in 2016 together with new versions of the 28 f2.8 Elmarit and the 35 f1.4 Summilux. All 3 lenses sported new (and much improved) metal screw in lens hoods, and the 2 28mm lenses had an improved optical formula which was designed to improve the edge and corner resolution by flattening the plane of focus.
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I wrote about this in 2016, and if you're interested you can read about it here‍

So this brings us to the version III, released on 26th October 2023
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The third version (Leica reference 11618) has the same 9 element in 6 group optical formula with 10 aperture blades, but there are a number of worthwhile changes.
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First of all the rear element has been adjusted to work as well with the modern M and SL sensors (notably the M11).
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The screw in lens hood has been replaced with a slide out lens hood (like that on the 35mm and 50 mm summiluxes. 
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Leica have added the close focusing helical to allow you to focus down to 0.4 metre. With an M camera this will need to be done either using Live View or FOTOS or the Visoflex 2. With the SL cameras you can use it directly with the EVF.  
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The maximum magnification of the old lens was 1:21.9, whereas the maximum magnification on the new 28mm is 1:11.3, hardly a macro lens, but still a definite improvement. 
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The new lens is 5mm wider than the previous lens and 1mm longer, but it is slightly lighter than the previous version with the screw in lens cap attached. At any rate, it handles very nicely on an M or SL camera.
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In the short term I had the lens I didn't have the opportunity to do any comparative testing with the previous version, but the optical formula is the same, so one would imagine that the performance of the new lens is just as good; the modification of the rear element suggests that definition should be even better with the modern digital M cameras.
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Whilst the new lens hardly qualifies as a macro lens at 1:11 magnification, it does make quite a significant difference, it's great for taking close up motifs and leaving the distance out of focus, and the 28 Summicron Asph does have excellent bokeh.
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This lens has three benefits over it's predecessor:
1. Close focus opportunities
2. Newly designed body with twist out lens hood
3. Improved performance with digital sensors
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The optical formula is the same as the 2016 lens (no bad thing). It comes in a black anodised version. 
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If you already have the 28 Summicron, then the new lens might not be a compelling upgrade, but Leica have made some real incremental improvements to what was already a great lens. If your principal camera is an M11 or an SL2, then the upgrade may seem quite compelling.
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The improvements on the new 28 Summicron are significant, and very much in concert with the recent re-issues of the 35 and 50 summilux lenses. This lens completes the remake of the 'holy trinity' of high quality practical lenses for the Leica M.
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