The Mullen is a strong bit which creates direct pressure on the tongue which will in turn create direct pressure on the bars. The Square Twist will create more pressure points on the mouthpiece also.
The 2 ½ ring gag, often known as a universal, is the softest version of this type of bit, as the ½ ring softens the leverage on the mouthpiece and poll.
Mouthpiece is sweet iron which has some oxidisation but this is completely normal.