From Urbanchicken

Mark1

The first version of the eglu was produced by Royal College of Art graduates, while no longer being produced by omlet some Mark1's crop up on e-bay every now and again.

Mark2

The Mark2 is physically bigger than the Mark1, and can be easily told appart from the Mark1 and the top of the door is much closer to the top of the eglu, the resign allowed for the keeping of larger chickens.

There are rummors of the Mark3 elgu which will be large still and be able to accomidate the largest of chickens (e.g. Buff Orpington).