(You may wish to read my series on the original Adventureland before this post.)
The manual informs us:
“SPECIAL SAMPLER” Never tried ADVENTURE? This special inexpensive sampler complete with 3 Treasures is a cut-down version of our large Adventureland. Guaranteed to supply hours of enjoyment: Try an ADVENTURE today!
Somewhat less kindly but still accurately, R. Serena Wakefield calls it “An abbreviated version of the full Adventureland, probably one of the first examples of crippleware in history.”
In essence, it is Adventureland with a.) no dragon eggs b.) the fish is just a normal fish, not golden and c.) the lamp is removed so the part of the game where you are supposed to go underground just cuts off.
This experience might be very disorienting as an introduction to adventure games, because many of the things that are part of the main game are left in! Previously essentially game items are now red herrings. There is flint and steel, skeleton keys, a bottle of water, mud, swamp gas, and the dragon from the original game (without the dragon eggs).
Even stranger: the opening part of the game has you climb a tree to find the skeleton keys, then chop the tree down and enter the stump. In the regular game, if you chop the tree down first, you hear something falling in the swamp; there’s a definite signal something “went wrong”.
TIMBER. Something fell from the tree top & vanished in the swamp
However, since the skeleton keys just open the door to the dark area, they are entirely useless in this game! So the signal that the player did something bad is misleading.
While many games genres have demos, even today, adventures never seemed to pick up on the trend. (I believe some of the Myst games had demos, but those are the only ones I can think of off the top of my head.) It’s just too hard to figure out where to cut. Also, one of the primary aspects of an adventure is to seed information and items in the early phases of a game where their presence may just be confusing in a demo.
Anyone recall any other particularly prominent adventure game demos?
EDIT: Some demos mentioned in the comments:
Space Quest 6 demo (This one is essentially a new mini-adventure to go alongside the regular one.)
Full Throttle demo (This includes 3 separate sections from the regular game.)
Mini-Zork (This was released relatively late in Infocom’s history, 6 years after Zork I.)
Day of the Tentacle Demo (Not exactly a demo, more of a preview video.)
Secret of Monkey Island demo (Also apparently a mini-adventure, as opposed to part of the regular game.)
From the Internet Archive.
Renga in Blue 
Space Quest 6 had a demo, which was more of a separate, short game than a cut-down version of the real game. In addition, I think Full Throttle from Lucasarts had a demo version.
Infocom had the Mini-Zork for C64, The Secret of Monkey Island had an excellent demo which actually offered a whole new (small) game using the assets of the full game… Lucasarts’ non-interactive Day of the Tentacle demo was also epic and legendary in its day.
I have added links to the demos you mentioned in the main post.
Back circa 1996 when I was making games for my own amusement with AGT (the Adventure Game Toolkit), I remember making a “demo” that would play for about 20 turns and then abruptly drop the player back into the main game. Sort of a “coming attractions” kind of thing. And I must have gotten that idea from somewhere; maybe one of David Malmberg’s AGT games had such a demo, or maybe it was suggested as a possibility in the AGT documentation. I’m pretty sure I didn’t invent the idea independently, and I definitely hadn’t played any of the above (non-AGT) games back then.