BTW: There is another hint in CAC 155/RFC 681. The line on page 2 that reads: "since the user is allowed only sixteen open files." My memory is V6 allowed more than 16, over 20 is my memory; but we would have to look at the structure to see what it is defined as.
Looking at the source in the archives for V5, we see that param.h has "#define NOFILE 15" and for V6 we see "#define NOFILE 15 /* max open files per process */". V7 has "#define NOFILE 20 /* max open files per process */" though, so maybe you are thinking of V7 bumping the limit to 20? Or maybe it was a local change for MIT, since param.h could be edited... But in any event, I think this means that the CAC 155 reference to 16 files just means V6 or earlier.
However, I just noticed there's more direct evidence for it being based on V5. On page 2 of CAC 155 we see
"For further information concerning the
different I/O calls the reader is directed to The UNIX
Programmer's Manual, fifth edition, K. Thompson and D. M.
Ritchie, June 1974."
BTW, CAC 155 is the PDF we have linked from the early network page. I hadn't noticed before now, but seeing the missing page refreshes my recollection.