Congrats on your new 7.3cm JM ;-) It looks like a keeper!
It indeed fits beautifully with the 5cm Summar.
Erik, this all black Hektor is just a thing of absolute beauty!
It indeed also looks entirely different mechanically as the more common single helicoid samples (distance scale integral part of the focus ring assembly rather than a separate component by the looks of it).
Now I understand why these all black samples are commanding such nauseating collectors prices (just a few weeks ago I saw one of these on the auction price for the asking price of a new digital M body).
This will make a very, very pretty lens paired with black paint bodies indeed.