So, a 6th level fighter gets two attacks now. If Angus also get's Improved Shield Bash does that confer a third attack?
Is it two sword strikes and a shield bash? Or is it sword & shield strike w/ less penalties?
Nope.
He can make an extra attack by two-weapon fighting, though... Which comes with horrible penalties unless you have the Two-Weapon Fighting feat.
All that Improved Shield Bash does is let him keep his shield bonus to AC when he attacks with his shield.
You can make two attacks, and one uses a lower bab. If one of those two attacks is with s shield, you can still get your shield bonus to AC.
You have two attacks. With the option for another attack by using the two-weapon fighting rules, with the penalties and costs that go with it (and yet another attack from the feat Improved Twoweapon Fighting).