let's clarify.
Bike is CB550K1 original US model, imported from USA.
Diagram I posted is from this model and is correct (I'm 100% sure)
Issue was that there was no spark (no power on Bk/W wire on coils) and horn did not work too.
Everthing else looked to be working properly, incl idiot lights, flashers, starter was turnong full speed, rear brake light worked... (headlight was not tested however)
After tracing issue down from coils through kill switch (no power coming from headlight) into headlight case termination points, I found out that only one of 2 Bk terminals have full power (12V). Other one has less voltage by approx 1.5V or 0V (was unstable). So, I connected all Bk wires to working terminal and all except horn works, engine runs w/o any issue.
Horn has similar problem, Bk wire that goes from junction point from main loom shows the same behavior and Horn does not work. When I connected +12V directly from the battery horn works. It confirms issue in Bk line going from loom to Horn.
So my conclusion, 2 Bk wires that are coming from junctions I mentioned (1 in headlight case, 1 to horn) has some connectivity issue most likely around these junctions. Looks like semi-cut wire, bad soldering...
As all Bk wires are powered by same lead from IGN switch and fuse (red wire), there is no issue in fuse box or IGN switch...
I apologize if I'm not very clear, I'm not English native speaker