Serbian M2F Gas Mask Size Medium. This Gas Mask uses 40mm NATO filters. NOTE: Even though it has the “S” symbol on the gas mask for it’s size, its in Serbian language. It stands for “Srednja”, it means “Medium” in English. Serbian M2F Gas Mask with It’s original carry bag, filter, and accessories. Serbian Military M93 Parka and Liner (Size Large). US M40 Gas Mask Hose. US (Korean War Era) Gas Mask Hood.