Bandai have already got a 1/144 Falcon from the film coming out soon
http://bandai-hobby.net/sw/jp/products/millenniumfalcon_landocalrissian.html
Great; I had to slog through 7 pages of 'It's gonna be great' vs 'It's gonna suck' to see if anyone had already commented on the detachable part between the Falcon's mandibles, as in the Bandai model that OSHIROmodels linked to.
Turns out, nobody did...
So I'm going to; my favorite theory is that it's sort of a connector, allowing YT series ships to attach to large containers, in order to move them:
I mean; as much I have always adored the Millennium Falcon and the Legends fluff behind the YT-1300 and all its derivatives and predecessors, I've always wondered how anyone in their right mind could be able to call these ships freighters! In the X-Wing and TIE-Fighter PC games, there were plenty of containerships, modular conveyors etc to see how a proper space faring transport should look, volumewise.
After all, even in the SW universe, where spacefaring is as common as atmospheric flight is to us, it makes no sense not optimising transport capacities; space travel may be common, but it still won't be cheap! So having a 25T to 100T internal cargo capacity on a ship with even the power to weight ration of a stock YT-1300 (let's assume there actually is such a thing
) would be incredibly inefficient. I mean; current trucks and trailers are allowed to weigh up to 50T here in the Netherlands, which roughly translates to a cargo capacity of nearly 40T, already more than the lower end of a YT-1300!
Therefore, should a ship of such capabilities be able to push a much larger cargo container, it would make that much more sense.
All of this going with the idea that a ship is a dedicated cargo vessel of course. Should we look at, for instance, a YT-1300P, which is a dedicated personnel transport, things would be different. Living, sentient cargo (I'm going to refrain from using the word 'Human' of course) is much more valuable than bulk cargo and could therefore be transported in such an expensive vessel.
So; in my opinion, if a YT-1300 were actually a self-contained transport ship with no capacity for pushing along larger volumes of cargo, it should be considered a galactic equivalent of a smallish delivery van!
In the Legends universe, there are of course plenty of clues to this, as we only have to look at the YT-2000 or center cockpit versions of the venerable YT-1300, and it's clear that those will never be pushing containers. But then; those could be the 'P' (personnel) versions so often mentioned.
Other theories state that it's either a life pod (there was mention of a pocket-fighter docked between the mandibles in several sources, although it looked more like the Crysalis life pod from Alien. there even was a toy version I believe? I certainly cannot determine any cockpit windows, which seem to be standard in SW ships), an aerodynamic cover to improve atmospheric flight characteristics, something purely cosmetic, giving the ship a sleeker profile (which would fit with Lando's personality), or perhaps a seperate cargo pod (which could explain how Han and Chewie actually ejected Jabba's spice cargo).
What do you think?
+++EDIT+++ Having taken another look at the Bandai kit pre-production pictures, I fear my favorite theory is probably obsolete already: in the few pictures that show the detachable part between the mandibles, one can clearly see engine exhausts on the back of the object.
So it's more than likely a life pod or pocket-fighter after all. my money is on life pod though, because it looks a bit underwhelming as an actual fighter (no weapons, no cockpit)...