Bulgarian Шлем Маска 41у (ШМ-41у)
The Bulgarian Шлем Маска ШМ-41y was nearly identical to the Soviet Шлем Маска ШМ-41y, only lacking the Soviet factory stamp. Additionally, a small circular ink stamp can be found inside (or sometimes on the back of) the mask. This example is missing its BSS-MO-4u filter, and was made in 1979.
Шлем Маска ШМ-41y = "Helmet Mask ShM-41u"