Vintage percussion press - photo

[desbromilow]

OK - appropriate googling (and an really annoying youtube video) I now see that the this is the big half of a friction screw press - so in my understanding the piece missing from the top is the motor drive which moves the disk to make the ram move up and down. So the difference between it and a fly press is simply the drive to the screw.

what's the difference between percussive and fly press? (other than missing the top flyweights)
where would you prefer one over the other?