"co -l" for same file in multiple branches fails

[Jonathan Wistar]

Greetings,

    I'm on a SUN platform
which (suspiciously) has the various RCS utilities in /usr/local/bin,
although

        %
what /usr/local/bin/co
says:
        SunOS 5.4
generic July 1994
and
        %
co -V
says:
        RCS
version 5.7

I observe that if I check out the same
file (locked) in 3 branches,
the 3rd (locked) check-out doesn't
actually lock the file:

        %
co -l foobar.c

        %
co -l10.1.11 foobar.c

        %
co -l10.1.8 foobar.c            
   /* fails */

although it looks like it succeeds (based
on the output to one's screen), but rlog(1) shows that
you don't actually get the 3rd lock.

I realize, based on the date, that this
particular code is about 14 years old, but I'm not aware
that there has been lots of development
going on, since that time.

So, what do we know about this bug (if
anything), or where can I read up on "known RCS bugs and fixes"
(or the like) ?

Thanks.

-- Jonathan Wistar