My idea of fun! Ok, you've not shown the vise pulled from the base, but my guess is the hold downs bolts are counter sunk into the slot of the swivel base. I base my guess by all the holes I see in the table.....they don't look random.
I can only imagine the forces involved with a 16" compared to my 7"!
Yep c - bag had to take top of vise off, all 200 +lbs of it. Their were 4 countersunk allen heads in the lower half of vise.
I had wanted to slide it rearward 5 or 6 inches sinc it was give to me for removal a few years ago. Now I just knew how to use it.