ARPCDHG is right. In the research for my book about helmet markings (out VERY soon...hint hint) I found at least 3 uses of “FAP”.....First Aid Post, First Aid Party and First Aid Point...... ......obviously it’s hard to determine which “FAP” lettering refers to as things were often applied to helmets which weren’t always “official”....... ...and it’s not a “hat”......HELMET!!