The seat shell is the same as the Marathon. The harness heights are the same as the Marathon. If it's so similar to the Marathon, why did they call it the Roundabout50? I have no idea. Because Britax is crazy I think (why else would they have 9 convertible seats, but discontinue their boosters?).
There are 4 harness slots. Lowest being about 10 inches, and highest being 16-17 inches. There are 3 crotch buckle positions. The Marathon has 1, that is in about the same spot as the center slot in the RA50.
Like the Roundabout40, it does not have HUGS (the rubbery things on the harness). Those are only required on the 65+ pounds seats. Also like the RA40 it does not have harness covers, but does have a belly pad (weird, to me the harness covers are more important than the belly pad).
The harness adjuster is thinner than that of the Marathon. The RA50 does not have the lockoffs like all the other Britax convertibles. It has a LATCH strap similar to the RA40 (not the Marathon with bars), but it only loosens and tightens on 1 side.
I know the seat is now on sale at Babies R Us. I went into my local store and it was still in a box on the top shelf above the RA40s. I asked them to get one down, and they did, and opened it for a display model.