They have both , I bought the swivel base type for two reasons ,
1st they were about $10.00 a pair cheaper - have no idea why.
2nd where I launch I would have to take them out of the tracks if they didn't swivel or take a chance of hitting the post along the dock.
3rd if I had it to do over I would have gone with Great Lakes Planers if they have that type base , just way more adjustment for sure....