What is item "UMa5191"?

Question

I saw that item on this Serebii page while browsing around the Pokearth for Castelia City. Is that just an error on Serebii or is that item some scrapped, unreleased item? I remember seeing it on one of Castelia City's buyable item section and it costs 50 Pokedollars. I think its a mail, but after seeing that it has its own page and it doesn't redirect me to any mail page, I'm not sure about it anymore.

On the hyperlink above it can also be seen that it is only found in the Gen 5 games.

Answer 1

UMa refers to the abbreviation of the constellation, Ursa Major. UMa5191 in particular actually refers to an item in SWSH, not B2W2. The item being Dynamax Crystals, which are given through events that let the player spawn event Pokemon at Watchtower Lair. There are 300 different Dynamax Crystals, each with a naming format of a star, one of the abbreviations of IAU designated constellations, and a Harvard Revised number that identifies a star in that constellation in the Bright Star catalogue.

As for why its in B2W2 according to Serebii? Probably a mistake that is untouched due to its obscurity. ☆UMa4301 is also a page but does not include B2W2 locations. I've even tried typing a few other random names from the item list of Bulbapedia and they don't even have their own Serebii pages. Another note, only 10 of the 300 codes are even used, and those where mostly given exclusively in either Europe and Japan. Its just really obscure.

Sources:
Bulbapedia list of Items in Gen VIII that includes the items with weird names (These items do not appear in the Gen V list)
Dynamax Crystals: Bulbapedia / Serebii