In V7 the file system call works such that mknod(2) is used to create special files and directories, it does it a little different than you realize.  Generally, there is no need to use it to create a regular file, although you could.   To wit the man page says:

 

SYNOPSIS

mknod(name, mode, addr)

char *name;

 

DESCRIPTION

Mknod creates a new file whose name is the null-terminated string pointed to by name.  The mode of the new file (including directory and special file bits) is initialized from mode. (The protection part of the mode is modified by the process's mode mask; see umask(2)). The first block pointer of the i-node is initialized from add. For ordinary files and directories addr is normally zero.  In the case of a special file, addr specifies which special file.

 

 

If you look at the code in mkdir.c (V7 mkdir.c source) you see that what is being done is a mknod with addr=0 is being called and then the two links.  

 

 

. . .

if ((mknod(d, 040777, 0)) < 0) {        fprintf(stderr,"mkdir: cannot make directory ­%s\n", d);

      ++Errors;

      return;

 }

chown(d, getuid(), getgid());

strcpy(dname, d);

strcat(dname, "/.");

if((link(d, dname)) < 0) {

      fprintf(stderr, "mkdir: cannot link %s\n", dname);

      unlink(d);

      ++Errors;

      return;

}

strcat(dname, ".");

if((link(pname, dname)) < 0) {         fprintf(stderr, "mkdir: cannot link %s\n",dname);

      dname[strlen(dname)] = '\0';            unlink(dname);

      unlink(d);

      ++Errors;

}

 

Where as creating special files did mess with the “special file” bits in the mode. 

If you look at the code in mknod.c (V7 mknod.c source):

 . . .

if(*argv[2] == 'b')

      m = 060666; else

if(*argv[2] == 'c')

      m = 020666; else

      goto usage;

a = number(argv[3]);

if(a < 0)

      goto usage;

b = number(argv[4]);

if(b < 0)

      goto usage;

if(mknod(argv[1], m, (a<<8)|b) < 0)

      perror("mknod");