This is the MEV from the Zero-X die-cast model by Aoshima from 2006 as displayed on Hobby Japan's site.
I have one of these but alas the small round insert is missing.
I think its an escape pod and it fits into the round hole in the white square on top of the MEV. The one pictures is slotted in with just the blue circular top showing. Its spring-loaded.
Does anyone have a spare escape pod or know where I might get one?