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"

Similar masks in my collection:  NVA ShM-41M, Russian ShM-62u, Polish MP-6, Polish MUA, Polish OM-14, Chinese TF-1