M10 and Elmar 9cm f4 Collapsible

[Jannis98]

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A few days ago I bought a second-hand M10 from the Leica Store in Hamburg. But after 20 minutes the fun was over because the rangefinder got stuck and I could no longer focus properly. Now it's being serviced and I'd like to take the opportunity to ask a question about compatibility. 

I really appreciated the Elmar 9cm f4 Collapsible (vintage 1956 in my case) on my M3. I would also like to use it on the M10. 

Now I find the following on page 23 of the M10 (german) manual: 

Zitat

Important:
- Not usable:
- Hologon 1:8/15mm,
- Summicron 1:2/50mm with close-up focus,
- Elmar 1:4/90mm with retractable tube (manufactured 1954-1968)
- Some examples of the Summilux-M 1.4/35mm (not aspherical, manufactured 1961-1995, Made in Canada) cannot be attached to the camera or cannot be focused to infinity. The Leica Customer Care department can modify these lenses so that they can be used on the camera.

As I understand it, my lens is explicitly listed as incompatible. I understand that it is not allowed to be retracted and that is fine. However, when I measure it in the extended state, the part inside the camera does not differ significantly from lenses that are compatible. For example, the Elmar 5cm f3.5, which according to the instructions is only not allowed to be retracted. So why is the Elmar 5cm compatible while the Elmar 9cm is not.

Can anyone answer this question for me or perhaps even tell me about experiences contrary to the instructions?

Edited April 16 by Jannis98
Translated into english

[UliWer]

My collapsibe 9cm Elmar (old version) works without problems with the M10. Of course you have to keep the tube extended. Whilst other collapsible lenses may be collapsed it is impossible to so with the 9cm Elmar: other than everybody thinks it is not the length of the collapsed tube, but it’s diameter. It is too larger so it will hit the sides of the camera‘s „mouth“, so never even try to collaps it otherwise it will seriously scratch the inner parts of your camera.  

Edited April 16 by UliWer