Well your English is better than my Slavic or Cryillic will ever be. For now, I think the sideplated boxlock to have begun its life in Romania and passed thru the Cugir proof house, contact info?? -
http://www.arms.home.ro/arms/pg/index.html ,where they may have taken the "Y" for black powder proof out of the Russian proof rules. Then in 1958/1959 it passed thru the proof house in Belgium and I don't know if Belgium honors/recognizes Romania's marks. Let me search for my Romanian proof info. The marks might be Bulgarian but I don't think so.
Kind Regards,
Raimey
rse