Hi!
As Dwarf Supreme mentions I don't drop in to frequently anymore, but I do read from time to time and this picture caught my eye.
That model is, as L4 mentions a prototype. Its all metal and was never released. Back then in prelude to the release of Adeptus Titanicus the studio made a lot of all metal models. These models precede the final design so they look very different from the standard "beetleback" that would be the final design.
If memory serves this one was a "siege titan". Note the "proto-chainfist" in one arm and a "proto-powerfist/wrecking ball" type arm on the other. Note that the carapace overhangs on both sides of the head in contrast to the beetlebacks. Also no ranged weapons on carapace. It looks as if the armor is mainly frontal (since that would be were a siege titan would take the most hits). I remember a stylized representation of such a carapace in one of the old books or perhaps an WD article.
I also remember another such model with a battering ram type arm.
I remember a picture with this model, assaulting a wall. I have my original books and WD's in storage right now, so I cannot confirm. Also keep in mind that back then they did not strictly keep games segregated as they do now, so such photos do not necessarily have to be in a article or book dedicated to epic. 'eavy metal and golden deamon pictures appeared everywhere and many times included epic figures, particularly titans.
Siege titans were mentioned either in the Codex Titanicus or a WD article. None were ever released.
These are even rarer than the all metal beetlebacks that were used for the original AT pictures (I own one of those). To my knowledge they have never been up for grabs. Otherwise I would own it.....

Primarch