Improved Signature Weapon is supposed to add +2 AB, +2 <some type?> damage when the weapon is used. At the moment this damage type seems to mimic whatever is already on the weapon, or randomly applies based on what was previously equipped? There might be something wrong with it - it doesn't stack with the damage already on the weapon, so the bonus to damage doesn't apply.
Examples:
Cav Sword Greensteel with +4 Positive perm essence, equipped gives +2 physical as the bonus damage.
Cav Sword Mdam with +4 Positive perm essence, equipped gives +2 postive damage as the bonus damage (not stacking with the perm essence). If there's no perm essence, it gains +2 physical damage which I'm now pretty sure isn't stacking with the +6 mdam physical.
Cav Sword Mdam with +4 pos perm essence, equipped after holding a cav sword with +4 psychic perm essence gives the +2 psychic damage which does stack.
Anarchic Razor Longsword with 1d4 Entropy enchanted, it gains +2 entropy damage which isn't stacking.
Anarchic Razor Longsword with 1d4 Fire enchanted, gains +2 fire and doesn't stack.
Nodachi Profaned Decapitator with 1d4 Poison enchanted, gains +2 poison damage, the 1d4 damage corrupts on the weapon and only the 2 damage applies poison damage.
Nodachi Profaned Decapitator with 1d4 entropy enchanted, gains +2 entropy and doesn't stack or crit.
Nodachi Profaned Decapitator with 1d4 Psychic enchanted, gains +2 psychic and doesn't stack or crit.
Nodachi Profaned Decapitator with 1d4 Fire enchanted, gains +2 fire and doesn't stack.
Nodachi Profaned Decapitator with 1d4 Sonic enchanted, gains +2 sonic and doesn't stack.
SO! It looks like the way the +2 damage is being calculated or added is not working, and it seems to be more strange when that gets mixed up with the new damage types.