Yes, the extension bolts on top of the standard ballast box. Just taller - more space to fill - and HEAVIER!
The backhoe sits out there a long ways! Quite a bit farther than either a box blade or the ballast box. Gotta pay a bit more attention when that hoe is on the tractor. The cylinder rod for the bucket curl is a shiny, smooth "target" just waiting for a nick or gouge!
IMO - if you have a need for a box blade - and they are very handy to have... it's a better investment than a ballast box. You can dress up a ballast box - for shovels, rakes, axes and chains, straps - but it's the wrong tool for grading, leveling and maintaining trails and driveways! Plus they can do extra duty moving snow.
But... a good, HD box blade is expensive. Yes, the heavier ones do a better job - IMO.
Don't forget - proper ballasting saves your front drive from excessive wear and tear - as well as extends the life of your rear tires (always spinning when the rear end is too light).
AKfish
Very true about that back cylinder - really didn't think about that. I could definitely have the use for a box blade. I take it there's not much maneuverability difference between the box blade and rear ballast. If I get a box blade would you see any use for a ballast box as well? Really wouldn't want to have both of those expenses. And I did not know you could use a box blade for snow - figured they'd tear up the drive - I do have asphalt drive if that makes any difference. But could use it for that. I've found my 3120 is really light in the snow with just the back blade on and the box blade could work out well for the more weight. Would you recommend the hydraulic box blades? I've seen more of them in the weight class Deere recommends. Is there anyway to add weight to a box blade?
EDIT: Also I looked at my back blade and it is a RB1084. Anyone know if it's iMatch compatible?