The Melee Master SPA is detailed on pages 223-224 of the Corrected Third Printing of AToW, but also in Campaign Operations, BattleMech Manual, some Alpha Strike books and probably elsewhere by this point.
One of the writers has asserted one "punch attack" consists of one punch from each arm, so applying the AToW description of:
When executing a physical attack, a MechWarrior with this ability can deliver one extra punch, kick, club, or hatchet attack during a physical attack phase (so long as all other restrictions are met, such as not firing weapons in the attacking limb).
Would mean FOUR punches instead of three as I read it. This leads to two questions:
1) Have the subsequent iterations of the rules expanded the definition of "one punch attack" to mean one from each arm (i.e., two)?
2) Would this increase the XP cost of the SPA?
A third shot at a headcap (for 'mechs above 50 tons) strikes me as worth rather more than 150 XP, but that's just my opinion. And no, this isn't based on my dislike for Traits (and SPAs) with "odd" costs (which drove extra logic in my spreadsheet and complicates compatibility with other parts of the rule system).