ADD51-98 are Monsters packs, containing one of the variations for each pose for that code. The only exception is ADD87 Snyad, Meazel, Xvart & Mite which only had 1 of each creature, despite there being 3 poses for the Xvart, and 2 for the Meazel. They are probably the rarest to find as a result.
ADD51 Kobolds
ADD52 Goblins
ADD53 Orcs
ADD54 Hobgoblins
ADD55 Gnolls
ADD56 Bugbears
ADD57 Ogre
ADD59 Troll
ADD60 Norkers
ADD61 Giant Rats
ADD62 Shambling Mound & Shrieker
ADD63 Bugbear Tribal Pack
ADD64 Troglodytes
ADD65 Duergar
ADD67 Sahuagin
ADD68 Drow
ADD69 Githyanki
ADD71 Evil Fighters
ADD73 Carrion Crawler
ADD75 Gorgon
ADD76 Stirges
ADD77 Umber Hulk
ADD79 Mind Flayers
ADD80 Blink Dogs
ADD81 Owlbear
ADD83 Centaur & Satyr
ADD84 Beholder
ADD86 Minotaur
ADD91 Rust Monsters
ADD93 - The Undead 1: Zombie, Shadow and Ghoul
ADD94 - The Undead 2: Ghast, Wight and Liche
ADD96 Zombies