Thought I'd mention here that the OP was asking about the original shell, and that the Mashey and Bourne shells were much newer than that.

On Thu, Jul 14, 2016 at 4:49 PM, Peter Jeremy <peter@rulingia.com> wrote:

On 2016-Jul-15 08:36:56 +1000, Dave Horsfall <dave@horsfall.org> wrote:
>On Thu, 14 Jul 2016, Clem Cole wrote:
>> Could you be confusing the fact the true and false were implemented by
>> external commands in some early shell's

I was always amused by AT&T adding a multi-kB copyright notice to what
had previously been an empty file.

>And on the Mac and FreeBSD, they still are (as well as being builtins).

FreeBSD provides a convenient list of what commands are (currently) builtin
to the provided shells and available externally:
https://www.freebsd.org/cgi/man.cgi?builtin

--
Peter Jeremy