From their announcement:
Ironhalo has just released a new set of bases. We have made a new size of base to fit a gap in our range. This base is slightly larger than a 60mm round, but not so big as to overshadow the model. They are ideal for Attack bikes, larger models, and as an alternate flying base.
We have made a generic version so you can create your own design and have also added a version to 12 of our current ranges, with the remaining coming soon.