Stock 305
A graffiti-covered store with a wall of spray cans, named after Miami's area code. Rockstar shot it four times and explained none of it.
Stock 305 appears four times in Rockstar's Ultimate Edition press material with no explanation attached, which is unusual enough to be worth a page.
What the shots show
A shopfront and hanging sign completely covered in graffiti and stickers, in a building whose every surface has been painted. Inside, a wall of spray cans running floor to ceiling behind the counter, with a character standing in front of it.
The exterior shots put people on and around a car outside, and a customer leaning on the counter inside against more painted walls and murals.
The name
305 is Miami's original area code and one of the most-used pieces of local shorthand in the city, appearing on clothing, in music and on businesses constantly. Naming a shop Stock 305 is roughly as subtle as calling a London shop Stock 020, and it is exactly the kind of naming a real store in that city would use.
What it probably is
A wall of spray paint behind a counter is a street art supply shop, and those are genuine cultural institutions in Miami. The city's Wynwood district turned large-scale mural work into an economy, and the shops that supply it are part of that world rather than adjacent to it.
That connects to something already visible in the base screenshot set. One of the official Vice City images shows a street of large-scale murals under an elevated railway, which is Wynwood in everything but name.
The open question
Whether any of this is playable. Grand Theft Auto: San Andreas had a tagging mechanic, with a hundred pieces to spray across Los Santos, and it remains one of the most fondly remembered collectible systems in the series.
A shop that sells spray paint, in a city built on mural work, in a game that has already shown tattoo and vehicle customisation as businesses you visit, is at minimum a suggestive combination.
It is also possible it is a clothing shop that happens to be covered in paint. Rockstar has said nothing.
What is not known
Everything except the name and the contents of the shelves. Rockstar has not described Stock 305, placed it in a district, or confirmed that it is enterable.