According to the reference I have, the "10" indicates a manufacturer of; Arsenal, Kazanlak (previously named Durjava Voenna Fabrika), Bulgaria, when found on ammunition made after 1946.
OR it also could be Russian State Arsenal, location unknown. Listed here to avoid confusion with the previously mentioned Bulgarian factory using the same code. Found on ammunition made 1946 and earlier. Factory Ceased operations in 1946.
The "55" indicates year of manufacture - which would pretty much rule out the Russian State Arsenal.
Maybe not exactly what you were looking for but it may get you closer.
Rob