A2 Slavers Stockade: Advent of the Ability Check?

Is the dude on the right supposed to be Ogre?

As you may have heard, I've been reading up on the A-series lately and comparing what I read to my fading memories of the '80s. A2 Secret of the Slavers Stockade has turned out to be a bit of a reality check for me because none of it matches up to my vague memories of that spring day in 1985 when an invisible Blodgett [the halfling thief from the pre-jens] leapt from the wall of a cavern to backstab a lance-wielding goblin who was riding on the back of a warg--and failed miserably. He was late on his jump, made a flailing attempt at a backstab, landed on his face for 3 pts of damage only to get up just in time to get lanced by the next charging goblin for 12 points of damage. 

For the life of me, I can't find an encounter area in this mod that matches that memory. Is this another case of the Donjon Maestro taking liberties or am I just misremembering things?

Anyway, the purpose of this here post is really to point out an encounter area comprised of a chasm with a bunch of stalagmites sticking up from the floor. One can cross the chasm, the module informs us, by leaping from stalagmite to stalagmite, but the text states that you have to 

"roll equal to or less than [your] dexterity on 1d20 for each jump." 

I pose this question to you dear readers: published in 1981, is this the first appearance of a roll-under ability check in a D&D product?