7.92x57 BT3 question

Why does IAA website headstamp identifier say: " B T 3 Unknown Factory, Yugoslavia (actually Cyrillic letters VTZ )"? What is “unknown” about it?


So, let me re-phrase my question to get an answer. How many military factories are in Kragujevac?

Vlad, at least there is also “Remontni Zavod Kragujevac” code “P3K” (RZK).

And not to forget “AT3” (ATZ) which is the old name of “BT3” (VTZ).