Location: Rustboro City, Route 114, Mossdeep City
The Shiny Beldum distribution was live for the first two months of the release of Pokémon Omega Ruby & Alpha Sapphire. If you take this Beldum, or its evolved form, to Mr. Stone in Rustboro City, he will be shocked and begin saying how it must be one of a kind and how sublime it is. He ends up saying that he wonders how it'd stand up to Steven's own Beldum
In addition to this, Steven Stone's letter about the Beldum you get given in the game also now mentions a second, special, Beldum that he sent you.
If you speak to Steven in Mossdeep City with it in your team during the Delta Episode, while he is in the Mossdeep Space Center, he will reference how it reminds him of a time he faced off against Rayquaza, teamed with a young man and his black Charizard, referencing the Mega Evolution anime special.
Finally, in Route 114, if you show it to the Fossil Maniac, he will mention how Steven travelled as far as Kalos in a hunt for this special Beldum.