info rcsでレビジョンオプションの確認(rcs)

先にドキュメントを確認すれば良かったかもしれませんが、
info rcsでドキュメント確認できます。

takk@deb9:~/tmp$ info rcs
Next: Overview,  Up: (dir)

GNU RCS
*******

This manual is for GNU RCS (version 5.9.4, 18 January 2015).

   Copyright c 2010?2015 Thien-Thi Nguyen

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with no Front-Cover Texts,
     and with no Back-Cover Texts.  A copy of the license is included in
     the appendix entitled “GNU Free Documentation License”.

* Menu:

* Overview::            General purpose and information.
* Usage::               How to use RCS.
* Hacking::             With source, you can help improve things.
* GNU FDL::             Copying and sharing this documentation.
* Index::

～省略～

ブランチナンバーとレビジョンナンバーについても載っています。

   A "revision number" is a branch number followed by a dot followed by
an integer, and a "branch number" is an odd number of integers separated
by dot.  A revision number with one dot (implying a branch number
without any dots) is said to be "on the trunk".  All integers are
positive.  For example:

     1.1         -- revision number for initial checkin (typically);
                    branch number: 1

     9.4.1.42    -- more complicated (perhaps after much gnarly hacking);
                    branch number: 9.4.1

     333.333.333 -- not a valid revision number;
                    however, a perfectly valid branch number

レビジョン指定についても、載っていますね。

takk@deb9:~/tmp$ info rcs

～省略～

2.1.1 Revision options
----------------------

As to be expected in a revision control system, many options are of the
form ‘-FLAG[REV]’, where FLAG is a single letter (e.g., ‘r’).  If
ommitted, REV defaults to the latest revision on the default branch.  A
revision can be specified in many ways:

BR.N
     Straightforward dot-notation, where BR specifies the branch.

.N
     Like BR.N, using the default branch.

BR
     Like BR.N, using the a command-specific computation of N, given the
     current tip I.  For ci (*note ci::), N would be ‘I + 1’, while for
     other commands N would be simply I.

NAME
     This is the symbolic name of a revision, as assigned previously by
     a ‘ci -n’ or ‘ci -N’ command.

‘$’
     The command computes the effective revision by examining the values
     of keyword expansions in the working file.

For commands that accept a range of revisions, the syntax is generally
‘REV1:REV2’, i.e., two revisions (specified as described above)
separated by a colon.

ブランチナンバーはBR、レビジョンナンバーは、Nという表記で解説がされています。

BR.N
.N
BR

は、それぞれ、

-r1.5
-r.5
-r1

という指定ですが、この中に、

-r.

が含まれていません。
ドキュメントに書かれていないということは、使わない方が良さそうです。