What you need for the KM56 and other vintage KM mics is a "Madenschraube" now called a "Gewindestift" (could not find a picture without point and with reduced head. This screw has a 1mm deep slotted, threadless head section whose diameter is slightly reduced, compared to the diameter of the threaded section, so that it fits exactly into the hole in the housing tube when turned counterclockwise, thereby holding the tube in place
Last time I checked, Neumann still had a few of them. Send a nice email (and I mean NICE) to:
[email protected] in Berlin, and I am pretty sure help can be found.
If not, I have made them myself on the stationary Dremel drill press: get the right metric thread size, and cut it 1 mm down from the top with a diamond Dremel cutting wheel, to reduce its head slightly, then slot it with a thin metal saw, then cut its length to size.
Of course, this only applies to the spirited do-it-yourselfer!
Best of luck,
KH