Well, he drops from TreasureClass "Baal (H)" (assuming no quest-drop). That TC then contains 5 picks: gold, junk, good, equip and the essence. The probability chances are 5 for gold and junk, 3 for good and 1 for essence (?), and 52 for equip. So that's a 52/68 chance to drop an item in the first place, assuming NoDrop = 0.
The TC "Act 5 (H) Equip B" contains several picks, but most importantly it contains the next TC' down the line' (equip a), at a probability of 1530. The probability of weap87 is 2 and armo87 is 1. All in all, considering the other picks the chance to drop a TC87 item is 3/1566, so 0.196%.
Assuming he drops an item, that is.
Edit: this also does not take into account that he drops multiple items. This is simply the chance for an equippable item he drops to be TC87 (i.e armo87 or weap87).