Just to update more than a year later:
I finally found a suitable CRT replacement (154-0644-05) from Qservice *Greece) at a reasonable price.
After installation of the new tube, the trace is now very bright and well focused at all scan rates, down to 500ps/div. I had to wait over a year to find the CRT, but it was really worth it!
I think Albert Otten pointed right when he mentioned the classical double-peaking phenomena.
Cheers,
Max
