Overview Reviews Product Description Winterization Kit, Biological Mask: M4. NSN# 4240-00-006-50319. Use it during cold weather conditions--lower than -20°F (-29°C)--to prevent frost accumulation on the inlet-valve caps. Brand New. Product Videos Custom Field Product Reviews (No reviews yet) Write a Review Write a Review × Gas Mask Winterization Kit Rating * Select Rating 1 star (worst) 2 stars 3 stars (average) 4 stars 5 stars (best) Name *Email *Review Subject *Comments