There are so many variants of the 350-point Crowther and Woods Adventure that it might be considered the first heavily “modded” game.
I even have to preface by saying the 350-point version; Don Woods himself made a 430-point version in the mid-90s. (EDIT: Don Woods made his version in 1978, the 1995 release was just a port into C.) The number of points possible in a particular port is the “identifying marker” for an entire family tree.
I’m not trying every version — especially since some really are just straight ports — but I think it’s worthwhile to dive into Peter Luckett and Jack Pike’s version, because the original source code is dated 31 Dec 1978. Other sources indicate it was worked on until 1981. However, the 1978 source seems complete, and the availability date makes it the earliest mod of the 350-point version of Adventure.
I’m using this port from the original source although there is a z-code version. Some comparison indicates the versions are identical except for normalizing the old-school ALL CAPS style.
Replaying when I have truly detailed notes in the form of my old posts about 350-point Adventure feels like I’m playing a find-the-difference puzzle. I’ve taken my original maps as a reference and will poke carefully at each nook and cranny for extra rooms.
Outside. This map goes all the way back to Crowther’s version of Adventure, before Woods came along.
The time to a map break seems to be one step…
out
YOU’RE AT THE END OF THE ROAD AGAIN.
w
YOU HAVE WALKED UP A HILL, STILL IN THE FOREST. THE ROAD SLOPES BACK DOWN THE OTHER SIDE OF THE HILL. THERE IS A BUILDING IN THE DISTANCE. TO THE NORTH LIES A SNOW-CAPPED MOUNTAIN RANGE WITH PEAKS THAT RISE INTO THE SKY. TO THE SOUTH, PARTLY OBSCURED BY A THIN HAZE, LIES A WHITE FORTRESS WITH SEVEN TOWERS. BEYOND THE FORTRESS, SHIMMERING IN THE SUN, THERE SPARKLES A GREAT EXPANSE OF WATER.
…but other than that description being elongated, I haven’t found any changes yet.
Renga in Blue 
Having played a lot of these versions I was particularly impressed with Mike Arnautov’s Adv770 for which I did a lot of bug fixing. It has greatly expanded Dave Platt’s version and is in itself an extension of his own Adv660; it has 476 locations and you really can examine everything. There is also an intriguing Easter egg on the way back from the island which I finally stumbled upon.