I've attached a picture of a Delta RIB that was down at Southampton Dry Stack recently.
It isn't that good a picture because I was more interested in the seat than the attachment.
The rails are surface rails, for example in the link below.
https://unwinsafety.com/shop/rail-pr...face-rail.html
There were four rails fastened at 300mm centres and the seats have seat fixtures built in to them.
It does give a very flexible system.