...or unless the Family Tree got it wrong.
I've tried Adventure 551, and it most certainly contained some of the new areas that are named in the first post of this thread, including the Sandy Beach, Ocean View, Thunder Hole.
(Of course, these were probably in Long's initial 500-point version, hence appearing in subsequent versions that descended from it.)
Rick Adams's site appears to believe that the 501-point version - on your site, "ANON0501" - is in fact itself a David Long creation. Also, that Adventure 551 - "MCDO0551" on your family tree - is *also*, either wholly or partially, a David Long creation, and that McDowell was merely the person who ported it to compile with f77 or f2c, and released DOS and Amiga versions as well as the Fortran source code.
If this is true - and how he knows it, I do not know, you'd better ask him - then this might imply that Russell Dalenberg got a few things wrong in his family tree, or did not have access to certain knowledge that Rick Adams managed to find, and was (like all good historians) guessing to fill up the gaps.
In particular, if David Long produced a 500-point version (as the Family Tree suggests) *and* the 501-point version (as Rick Adams's research seems to suggest), the chances would seem pretty high that they are one and the same except for one of them containing the "Last Lousy Point" of the original game, and the other not containing it. (Thinking logically: perhaps he had not fully solved the original game when doing the 500-point version that got copied into the wider world, and put the Last Point in to bring the total to 501 when he finally found it.)
And if he was involved with the 551-point version too, as Rick Adams seems to suggest, then it would seem logical that the 751-point version descends pretty much from it. So really, if Rick Adams is correct, ANON0501 should be LONG0501 (essentially the same as LONG0500, or a descendant of it), and MCDO0551 should really be LONG0551, with the four versions descending in sequence rather than all being separate offshoots (and so the branch would be at the 551-level, with Long's 751-point version on one branch and Picton's more recent 701-point merger of 551 and 550 on the other.)