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Weapon and Armor Proficiency: Monks are proficient with the club, crossbow (light or heavy), dagger, handaxe, javelin, kama, nunchaku, quarterstaff, sai, shortspear, short sword, shuriken, siangham, sling, and spear.
Monks are not proficient with any armor or shields.
When wearing armor, using a shield, or carrying a medium or heavy load, a monk loses his AC bonus, as well as his fast movement and flurry of blows abilities.
Weapon and Armor Proficiency: A sohei is proficient with all simple and martial weapons and with light armor.
The latter overwrites the former (though this was not the archetype writer's original intention, but Paizo specifically acknowledged that this is how they want it to work) so you can Flurry while armored as a Sohei. The Ironskin Monk doesn't even touch the Weapon or Armor Proficiencies the Monk receives, so it does not benefit from that errata.