Go to google...seach for images for a "V2 mechbox" and "V3 mechbox". The outer shape, dimensions and visible outside parts will be different. There are differences internally as well depending on models.
Pre-built mechboxes are built for one model of AEG or another since trigger/selector plate/etc....will differ between builds. So a V3 that is used in a G36 is not the same as a V3 in an AK. Very similar...but not exactly.