Hey Greyman ,nice to hear from you. Sorry I missed this and have taken so long to respond. In my copy of the Hurricane II Manual it lists 2 mods for the glycol header armour. The first is mod no 188, Armour forward of header tank 8.08 lbs and Mod 207 Armour forward of header tank introduced 7.14 lbs. Unfortunately in most restored Hurricanes this armour has been removed. However Squadron Signals Publications has an excellent picture of it on page 40 of "Hurricane Walk Around". It looks to be between 4 to 6 mm thick. Additionally the Cowling from the base of the windscreen to the back of the engine covering both the auxiliary fuel tank and the glycol header tank was 10 swg(gauge) duraluminum (1/8 inch thick). This is covered in "Knights of the Skies" by Michael C Fox. Send me your email if you want me to scan the picture and send it to you.