The Russian/Soviet size 5 Маска Детский 1А (МД-1А) was the largest size mask included with the Противогаз Детский Фильтрующий Школьный (ПДФ-Ш) kit.  It was made of gray rubber and sized for older children.  This example was made in Ohrenburg in 1976.

Маска Детский 1А (МД-1А) = Child's Mask 1A (MD-1A)

Противогаз Детский Фильтрующий = "Child's Filtering Gas Mask"

Школьный = "School"

Similar masks in my collection:  Russian MD-1A size 1, Russian MD-1A size 4, Russian MD-3, Russian M-49