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Following the lead from this answer. And reading page 2-15 from the manual from Bitsavers (thanks ^@grawity).

Shared Data
The second design principle of the Domain/OS distributed file system, sharing by default, implies a global uniform name space. The name space of the distributed file system appears to users like that of a giant timesharing file system. It is a traditional UNIX hierarchical name space, except that absolute pathnames can begin with the name of the network root (called //). It is also possible to express pathnames relative to the root of the local node (the / directory).

There is also an older manual from with a "First Printing: July, 1985". On page 1-4:

The double slashes (//) in Figure 1-2 represent the top level of the naming tree, the network root directory.

So, we have confirmation that Domain/OS from Apollo used // for network root.

Following the lead from this answer. And reading page 2-15 from the manual from Bitsavers (thanks ^@grawity).

Shared Data
The second design principle of the Domain/OS distributed file system, sharing by default, implies a global uniform name space. The name space of the distributed file system appears to users like that of a giant timesharing file system. It is a traditional UNIX hierarchical name space, except that absolute pathnames can begin with the name of the network root (called //). It is also possible to express pathnames relative to the root of the local node (the / directory).

There is also an older manual from with a "First Printing: July, 1985". On page 1-4:

The double slashes (//) in Figure 1-2 represent the top level of the naming tree, the network root directory.

So, we have confirmation that Domain/OS from Apollo used // for network root.

In this answer:

However, some other Unix-like systems, for example Cygwin or the old Apollo Domain/OS, use the // prefix for network paths such as //fileserver/path/to/data. POSIX allows this as well

I made a comment there.

New data

grawity answered, he said this:

Bitsavers has a collection of old Unix manuals, including Apollo's – there should be plenty of examples there. The TUHS website should have something as well. – @grawity

I downloaded the Apollo manual from 1989 form here

Inside the manual, there is this description.

Shared Data
The second design principle of the Domain/OS distributed file system, sharing by default, implies a global uniform name space. The name space of the distributed file system appears to users like that of a giant timesharing file system. It is a traditional UNIX hierarchical name space, except that absolute pathnames can begin with the name of the network root (called //). It is also possible to express pathnames relative to the root of the local node (the / directory).

So, we have confirmation that Apollo used // for network root.

^{As there is additional info that has been found, I'll keep this for one day more. No need to up-vote (or downvote) this answer will go away. It is not mine, anyway, I am just providing a reference link.}

Following the lead from this answer. And reading page 2-15 from the manual from Bitsavers (thanks ^@grawity).

Shared Data
The second design principle of the Domain/OS distributed file system, sharing by default, implies a global uniform name space. The name space of the distributed file system appears to users like that of a giant timesharing file system. It is a traditional UNIX hierarchical name space, except that absolute pathnames can begin with the name of the network root (called //). It is also possible to express pathnames relative to the root of the local node (the / directory).

There is also an older manual from with a "First Printing: July, 1985". On page 1-4:

The double slashes (//) in Figure 1-2 represent the top level of the naming tree, the network root directory.

So, we have confirmation that Domain/OS from Apollo used // for network root.

In this answer:

However, some other Unix-like systems, for example Cygwin or the old Apollo Domain/OS, use the // prefix for network paths such as //fileserver/path/to/data. POSIX allows this as well

I made a comment there.

grawity answered, he said this:

Bitsavers has a collection of old Unix manuals, including Apollo's – there should be plenty of examples there. The TUHS website should have something as well. – @grawity

I downloaded the Apollo manual from 1989 form here

Inside the manual, there is this description.

Shared Data
The second design principle of the Domain/OS distributed file system, sharing by default, implies a global uniform name space. The name space of the distributed file system appears to users like that of a giant timesharing file system. It is a traditional UNIX hierarchical name space, except that absolute pathnames can begin with the name of the network root (called //). It is also possible to express pathnames relative to the root of the local node (the / directory).

So, we have confirmation that Apollo used // for network root.

^{As there is additional info that has been found, I'll keep this for one day more. No need to up-vote (or downvote) this answer will go away. It is not mine, anyway, I am just providing a reference link.}

In this answer:

However, some other Unix-like systems, for example Cygwin or the old Apollo Domain/OS, use the // prefix for network paths such as //fileserver/path/to/data. POSIX allows this as well

I made a comment there.

New data

grawity answered, he said this:

Bitsavers has a collection of old Unix manuals, including Apollo's – there should be plenty of examples there. The TUHS website should have something as well. – @grawity

I downloaded the Apollo manual from 1989 form here

Inside the manual, there is this description.

Shared Data
The second design principle of the Domain/OS distributed file system, sharing by default, implies a global uniform name space. The name space of the distributed file system appears to users like that of a giant timesharing file system. It is a traditional UNIX hierarchical name space, except that absolute pathnames can begin with the name of the network root (called //). It is also possible to express pathnames relative to the root of the local node (the / directory).

So, we have confirmation that Apollo used // for network root.

^{As there is additional info that has been found, I'll keep this for one day more. No need to up-vote (or downvote) this answer will go away. It is not mine, anyway, I am just providing a reference link.}