Gate valve type Z40Y, Z40H letters - what does that mean?

Stainless Steel Strainers High Temperature Check Valve Ductile Iron Poles Ductile Iron Vs Cast Iron Valves Cgi Cast Iron Cast Iron Vs Ductile Iron Valves Pearlitic Ductile Iron Cast Iron Ferrous Broken Cast Iron Annealing Cast Iron Adi Cast Iron Sg Cast Iron Grades Cast Iron Metallurgy Cast Iron Vs Iron Cast Iron Case American Ductile Iron Cast Iron Sphere Mottled Cast Iron Cast Iron Nickel Cast Iron Hanger