I don't know all of the differences now as I've never compared them in person but the pickups are all designed differently. I can't say they're "Updated" But are just different models. The Ventures I/MK I was still considered the top of the line model in the 60s (Not counting double-necked models as that's different) and the Ventures II/MK II and MK V models were cheaper models.
This thread that Shawn posted might be helpful; He compared his Hallmark guitars instead of Mosrites but two of the guitars are a Hallmark 60 Custom (Ventures I/MK I style guitar) and a Hallmark II (Ventures II Slab Body/MK II Style Guitar) And they are very close copies.
viewtopic.php?f=31&t=5789I also demonstrated the tone of my Ventures II (Same as a MK V minus the name) In a video seen in this thread:
viewtopic.php?f=6&t=5765I did have the amp settings as such: Mids and bass turned as low as possible and the treble and presence set as high as possible. I also had what I think was a set of 9s on the guitar.
Hopefully the video suggestions will help give you a general idea of how the guitars sound.
Austin