The STOLL FS is also a grenade launcher, hence the ribs and length.
It's open on the end in order to be a wire-cutter.
Don't know which is best.

As to indexing, only one that that is important for is the short Belgian combo device, and then only in conjunction the the South African mfd compensator add-on.  Otherwise, I don't think it matters, as I have not seen any that seem to require indexing save the example I mentioned above.  For that unusual example, I'll just make a washer, and thin it out to suit.  AR washers  would require reaming the center to fit the FAL muzzle.