Igniter AN-M5, a component of the 500 lb Incendiary Bomb AN-M76, consists essentially of two concentric steel tubes welded to annular ends. One end ring has two filling holes which are closed with pipe plugs. These pipe plugs must not be tampered with or removed.
Bomb, 500 lb Incendiary, AN-M76
OP 2216, Volume 1 - Aircraft Bombs, Fuzes, and Associated Components (1960)
TM 3-400, Chemical Bombs and Clusters (1957)