I've got one of those tillers, but unfortunately don't have that same attachment.
My first thought is that it would attach to the depth shoe somehow utilizing that large hole in the wings...but that is only a guess. I haven't seen one of those furrowers in person. I can't find any images online that show what you are looking for, either.
You might find someone that has the answer over at MyTractorForum.com. They've helped me out a lot over the years with questions on lawn and garden equipment.
Alternatively, the Ariens company (who built the Rocket tillers) is still around. It might be worth contacting them directly to see what help they could offer.
I'll keep looking around...and if I find something I will pass it along.
