check out old revision 

co -l[rev]

-l is the same as -r, except that it not only retrieves but also locks the retrieved revision for the caller.

Nok to co old revision and ci as head 

$ co -l1.1.1.1 mencvcd
RCS/mencvcd,v  -->  mencvcd
revision 1.1.1.1 (locked)
done
$ ci -l1.2 -m'- co from my v1.1.1.1. cosmetic changes' mencvcd
RCS/mencvcd,v  <--  mencvcd
ci: RCS/mencvcd,v: no lock set by tong for revision 1.1

documented on: 2005.08.04