This has long been a bug-bear of ours that these schemes just disappeared - well now according to Bell of Lost Souls, the new Horus Heresy: Inferno book will include a glimpse of this long lost scheme.
Sure it's a little bit different, but there it is - plain as day - the original "M" Legion icon.
The Thousand Sons went through many incarnations to be fair, even Jes Goodwin had his own scheme with the "M" inside two planetary bodies in orbit with one another. The "M" was the defining point of unity so we're super excited to have this back as part of the fluff - let's find out more and see where this leads!
*Keen-eyed observers will also note the GIJoe tribute on the chest in the form of Cobra Command fangs*