Hehe
A bit of a clarification: Viator actually gained that weight in about a one month period, and it was more like 63 lbs. IIRC.
Jones called it the "Colorado Experiment." It's pretty controversial. There's lots of hearsay about how exactly Viator trained during that period; the HIT camp claiming it was strictly the result of Jones workouts, but others (Jeff Everson comes to mind) say Casey would sneak off and do volume training at a nearby Gold's (though, interestingly enough, Jones and Darden maintain there was NO Gold's within driving distance).
What we DO know is that Casey was artificially light at the start of the experiment. He had been terribly ill beforehand, and Jones put him on a restricted diet to lose even more muscle. When they started the experiment, they fed him something like 6-7,000 kcal/daily and trained his ass off. Thanks to muscle memory he blew up like a balloon.
As far as Arnold goes, he was regaining lost muscle, too. I dunno if he "only" gained 25 lbs. as Mentzer claimed, nor can we really compare Jonesian HIT and volume training given the fact that we're dealing with two different guys. (Arnold was the superior bodybuilder of course, but it's entirely possible Casey could gain a lot faster, regardless of the training methods employed.)
All that aside, the Colorado Experiment has _nothing_ to do with Mentzerian HIT. Michael brought it up in HD1...why, I'm not sure *shrugs* Doesn't matter.
I will say _this_, though: it's a bit hasty to blow off Heavy Duty altogether. Mind you, the latter-day Mentzer stuff was...hmm...how to be diplomatic here?..."out there"?
Yep. It was. I was friends with Michael, but come on...training a bodypart once a month? He simply became more and more out of touch, absolutely determined to prove that his "theory" fit the bill. Anything that got in the way was angrily dismissed
BUT, his earlier stuff is actually quite good. The Dorian Yates-style, train-each-bodypart-once-a-week routine in HD1 (1993) worked extremely well for me, and even earlier, higher-frequency Mentzer routines might have more promise.
In fact, some of them remind me a
bit of Dante/DC's training, albeit with more volume (up to 8 work sets, though usually more like 3-6).
But Slothy, I gotta go with Ghetto and Beast here. The information at Michael's site is all about his latter-day ideas which, God rest his soul,
are fraught with problems

You'd be better off looking into 5x5, DC, HST, WSB or any of a half-dozen other routines described in detail here.