I am interested in pulp-era armor -- not unusual or outlandish armors, but kinds that would be relatively widely available. I had thought that very little armor was worn from early in WW1 (or earlier) until the invention of kevlar. But there are bulletproof vests in GURPS Cliffhangers ("PD 3, DR 4") and GURPS High-Tech ("PD 4, DR 6" for TL 6, which represents 1900-1950). Cliffhangers also has a concealable metal chestplate ("PD 4, DR 10"). And of course there are also various helmets and articles of heavy leather clothing. Call of Cthulhu's 1920's Investigator's Companion has no armor, though that could be as much due to the style of the game as to the history of the era.
So here are my questions:
1. What are PD and DR in GURPS?
2. How would you translate the above armors into HERO terms?
3. What can you tell me about the actual history of metal breastplates, pre-kevlar bulletproof vests, and the like in the 20th century?
4. What armor values (in HERO terms) would you give for WW1 and WW2 helmets?
5. What armor values would you give for clothing like heavy leather jackets? Would you give them a point or two of PD, with the limitation not vs. firearms, or would you handle them in some other way?
6. What else can you tell me about this general subject that I did not ask about specifically?
Direct answers or suggested references for any of the above questions would be greatly appreciated.