There is indeed such a beast. Once upon a time, Cold Steel sold an Emperor Collection (as Jlauffer said). It included a Mini Tanto, A standard Tanto, and a Magnum Tanto. This is the only time and place that I know of where the Magnum Tanto had the brass fittings.
IIRC, they were all the mono steel variants, not San Mai III. ((
Jlauffer's correction: Referencing his post on the next page, the Emperor Collection was unique at the time in that all the blades were San Mai III. This is the only instance of San Mai III on the Mini Tanto with brass fittings, and a raised spine on any Mini Tanto ever. It is the only instance of a raised spine on the standard-size Tanto, and the only instance of brass fittings on a Magnum Tanto.)) "Emperor Collection" is clearly marked on one side of the ricasso for each knife.
If you search BladeForums, you'll find a thread with only two posts in it, the start of it being a picture of the Cold Steel Emperor Collection, as it was shown in a brochure/catalog. In this picture, you'll clearly see that while the Tanto and Mini Tanto have brass fittings, the Magnum Tanto has stainless steel fittings (as we are all familiar with). As far as that is concerned, it would indicate that there have been two "generations" of the Emperor Collection, and that the whole notion of a Magnum Tanto with brass fittings wasn't around for a long time, though the knife did indeed exist. ((
Jlauffer's correction: The picture in that particular thread isn't a true Emperor Collection. If you reference his post on the next page, you'll see that it is similar, but not exact pieces, "cobbled" together to resemble the Emperor Collection. The "Emperor Collection" marking is also missing from the blades in this picture.)) The Magnum Tanto with brass fittings was only ever available as part of the Emperor Collection, never on its own. You won't find a Magnum Tanto with brass fittings
without the "Emperor Collection" marking on it, unless you happened to come across some rare Magnum Tanto prototype with brass fittings, if such a thing even exists. I do not know how long the Emperor Collection was sold, but it wasn't for long, I assure you.
As for the balance of the knife, what Jlauffer said is correct. The Magnum Tanto with brass fittings from the Emperor Collection has a sub-9" blade. Therefore it is much closer in size to a modern Magnum Tanto IX, rather than a 7.5" blade Magnum Tanto II. While brass is slightly heavier than steel, and could likely affect the balance of a Magnum Tanto II, the extra ~1.5" of blade on the original Magnum Tanto would no doubt make it tip towards the blade side of the knife, regardless of brass or stainless fittings.
For a final note, it is true that there was a 7.5" blade tanto called the "Magnum Tanto". This was the earliest version of the Magnum Tanto II. It was identical in all but the name, and the fact that it was mono steel, and not San Mai III. This knife came to be when they phased out the sub-9" blade original Magnum Tanto and replaced it with a 7.5" blade tanto bearing the same name. However, very shortly after, this model was given a San Mai III blade, renamed to Magnum Tanto II, and the San Mai III 9" Magnum Tanto IX followed it. Every 7.5" tanto variant, regardless of its name, has had stainless fittings (I am not sure if the fittings on the 3V variants are stainless, though I can tell you that they are definitely steel, not brass).
