Mr. Leonid Sinitsin, the one and only artisan able and skilled enough to restore any DST-design cartridge. Just in case... as a plus, he hand-made quite interesting more conventional cartridges, as well.
Just don't ask for a direct-sale of Tzar's;-) - its worldwide sole-agent is Robyn Wyatt, in the USA.
I repeat: above is a serious advice, so please don't ask for!
Interesting manufacturer, indeed.