old time signatures

[Juergen Reuter]

Hi, Mats, Christian, and all others on the devel list!

Long ago when lily 1.1.55 came up, mensural time signature styles
old2/4, old6/8alt, old4/8 were added (by Mats & Christian, the
changelog says), using a three quarter circle opened to the left side
(like the letter "C", but reversed on the x axis).  My resources (MGG,
Harvard Dictionary of Music, DTV Musikatlas and others) show for (white)
mensural notation only a single form with such a reverse "C", namely
the "tempus imperfectum cum prolatione minori" typus with brevis as beat;
i.e. the reverse "C" equals the vertically slashed "C" (alla breve).

I guess, your resources refer to some notation that was used earlier,
maybe franconian ars nova or something alike.  Could you check this and/or
do have you any bibliographic or WWW references?

Thanks in advance,
Juergen