The 424mkII was a particular Portastudio, & a pretty nice one.
For each particular Portastudio model, there are no variations, but of course there are numerous models.
It's true, that for the 424 itself, there were 3 variations, being the "standard", the mkII and mkIII, but each mkII is like every other mkII out there, design-wise.
There are no variations within a particular model.
The 424mkII, on it's own, is a pretty nice unit. It's an 8x2 mixer with 4 full mixer strips, integrated 4-track cassette recorder, 3-band EQ w/mid-sweepable, records 2-simul in buss mode & 4-simul in direct mode, etc.
I like the Tascam 424mkII Portastudio. I've done a lot of recording on the 424mkII, so I know it pretty well. It's one of several of my favorite Portastudio designs. I actually have two 424mkII's. I bought one brand new in '97, and then picked up a second one in 2001, used off Ebay.
Anyway, if you favor 4-track cassette Portastudios, the 424mkII's a nice unit to have, especially if you can find one in good condition & low bid price. Not available as new any more, except in the rarest cases of "new-oldstock", which you may find once in a blue moon, but much less incidence of that as time goes on.
I guess this is my cue, see: Tascam 424mkII Portastudio