"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