There is an excellent discussion on the "frein harmonique" in Audsley's
book, "The Art of Organ-Building" (see vol II, p. 473).  I have not
really followed the discussion of the frein in recent MMD's but I thought
I remembered something about this from Audsley's book(s), so I revisited
the well-worn bound volumes and found some superior diagrams of different
applications of the frein in pipe building.  The diagram on p. 473 shows
an adjustable frein, a "T" shaped metal device, held in place with two
screws.

I may look like a complete idiot sending this note -- sorry if this has
been covered already or is completely irrelevant.  As I said, I haven't
been following that thread of discussion.

BTW: Audsley credits Gavioli for the invention.

Steve Matta