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The “IB” designation stood for incendiary bombs, either the magnesium AN-M50 series, the jellied gasoline AN-M69 series, or the jellied gasoline AN-M47 series. The “GP” designation stood for general purpose bombs, which represented the largest percentage of ordnance dropped on Germany during the war. GP bombs had casings of moderate thickness coupled with an explosive charge that weighed up to 50% of the bomb’s weight. The bombs ranged from the 100 pound AN-M30 to the 2,000 pound AN-M66, although the bulk of GP bombs dropped in Europe were the 500 pound AN-M64 and the 1,000 pound AN-M65. The “HE” designator is a bit of a puzzle as the GP bombs were sometimes termed HE or High Explosive. A possibility is that HE refers to the 4,000 pound Light Case bomb, the AN-M56. In the Light Case bomb, 70% of the bomb’s weight is represented by the bomb’s HE filler.
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Many thanks for this, A.J.--a huge help.