There are three versions, actually.
27F is the "700c" model. It has a small platform, and the mounting struts at the bottom are angled. 27R is the matching rear.
F20 is the "26/Mt. Campee" model. It has a large platform (at least 30% larger than the classic), and the mounting struts at the bottom are angled. The R20 is the matching rear partner, but it has no lowriders. It's closer in design to the Riv 32R.
??? is the "650b" model, sometimes listed as a 26" model. It has a small platform and the mounting struts at the bottom are straight. There is a matching rear, also called "650b".
The angled struts may reach to the rear eyelet on a fork, but the straight struts are meant for front eyelets only.
I mounted the "650b" model with straight struts on my girlfriend's San Marcos, and I had success in using the Tubus p-clamps (2-bolt) for the lower mount, and the Tubus fork clamps for the upper mount.
The "700c" 27F may ride quite high above the tire on a 700c Atlantis. You may be able to lower it by mounting it to the rear eyelets. (in that case, you may want to use spacers so the rack legs don't hit the fork ends)
But it will fit..
Be aware that the lowrider frames are shorter and wider than most modern panniers. The top rail of the lowrider frame is lower than the top rail on the Blackburn/Tubus style lowrider racks, so a modern pannier may hang low off the bottom of the lowrider frame. (On my gf's SM, I mounted small/medium Mountainsmith panniers, and the bottoms dangle a couple inches past the lowrider frame).
I recently picked up a F20 Mt. Campee off the list, and am tempted to buy the Ostrich DLX panniers that are designed for these racks.
Cheers,
Tim