I think the box will contain an ornate dog whistle. Blow it once, Leman Russ will spring out of nowhere, drop a stick at your feet and run rings around you. If you rub his tummy and scratch him behind the ears, he will sniff out the other living primarches for you. He might even dig up the dried bones of a few of the dead ones if you are not careful.
That is the optimistic version. In the ironic version, the box contains the smallest ever squad playing the Imperiums smallest violin.