One of the particularly interesting things about AD&D morale is that the modifiers given in the Monster Manual (plus those gained from PHB spells such as Bless) have
no relation to the actual morale system, which was a % system!
It's particularly interesting because the original D&D morale system was that contained in Chainmail, which was an even odder system - in fact, three systems (
see here).
It seems that the PHB and MM of AD&D are actually using the morale system that would be given in Moldvay's Basic D&D (Holmes' edition has references to morale, but no system).
In fact, the AD&D rules for morale were of the sort that in a fight against (say) 12 goblins, the entire band of goblins would likely flee after a couple of them were slain. Five goblins slain? Automatic fleeing for the hills. I don't think I ever used the AD&D system much. Meanwhile, I used Moldvay's system extensively.
AD&D 2E's 2-20 scale seems more reminiscent of Moldvay's 2-12 scale.
Cheers!