Update
In the days following the original story, which you can find below, the Call of Duty: Vanguard Internal Alpha known as Slipstream has also been discovered on the back-end services of the PlayStation 5 Store. It has a total download size of 35GB, which as expected, indicates that this is not a full or final build of the game.
In March, rumors began to surface from credible sources Eurogamer and VGC which reported that Sledgehammer Games are to release Call of Duty: Vanguard in the fourth quarter of 2021. Since then, further details for the title, including its WWII time period and substantial Warzone integration, have slowly come to light. Despite the public knowledge of the title, and Activision recently confirming that the San Francisco based studio will in fact develop this year's title, no visual assets relevant to the project have surfaced.
On the evening of June 29th, however, fans have gotten a first-look at what appears to be the logo for an Internal Alpha build of the title. Referred to as Slipstream, assets relating to the build have been discovered within the Battle.net client by BlizzTrack.
Slipstream
As is expected of this time of year, the latest Call of Duty title is undergoing extreme testing conditions internally. An Internal Alpha, also known as a Friends & Family Alpha, has been added to the Battle.net services in order to allow Developers to playtest the latest builds of the game. Similar to last year's The Red Door, Slipstream is being used for Vanguard.
To make things clear, here's a breakdown of the confirmed identities for the title...
- Release Title: Call of Duty: Vanguard
- Internal Alpha Title: Slipstream
- Project Name: S4
- Alternate Project Name: Fore

It's important to note that, at the time of writing, Call of Duty: Vanguard is expected to remain the title that the game is given upon release. Slipstream is simply a name used for the unfinished product which was never intended to become public knowledge. This is a common practice which is believed to prevent leaks, but as proven by the last two years, is not effective.
The implications of the name Slipstream are currently unknown. While some have pointed to the Slipstream WWII radio drama, it's unclear whether or not it is related at this time. Although The Red Door was a direct reference to a Campaign mission found in Call of Duty: Black Ops Cold War, previous Internal Alpha names have not always been as relevant.
To learn more about what's currently believed to be true in regard to Call of Duty: Vanguard, refer to the following article.

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