Some may not realize it but there are at least three kinds of 
"Mandolin" or "Banjo" piano effects that I've seen over the years.

The most common one consists of small metal pieces fixed into the end
of a fabric that is in turn fastened to the actuating rail.  When
"turned on" the rail drops and the metal pieces are in a position where
they can be struck by the hammer and driven against the strings for
that note.  These are probably the cheapest to produce but they are
probably also the hardest on the piano hammers.

There is another type that has the small metal tabs fastened to a piece
of fairly rigid material, and the Mandolin effect is obtained by the
vibration of the strings against these metal tabs.  In this type, the
hammer does not hit anything other than the strings as it normally
would.  The shortcoming in this type seems to be that the material
holding the metal tabs is required to hold them in contact with the
strings, but the material often has lost its rigidity and
effectiveness.

The third type that I've seen consists of a metal rod which has small
metal balls on the ends of spring wire running nearly the full length
of the rod.  When the rod is rotated the spring wires hold the balls in
contact with the strings so that when the string is struck by the
hammer there is the [buzzing] metallic sound, sort of like a Mandolin,
or Banjo.

These three types are all of the on/off type so that the operator can
bring on the effect at will.

There is a fourth method that I've run into over the years that some
people seem to delight in.  That is, pushing thumb tacks into the face
of the hammer so that the metal head of the thumbtack actually does the
striking of the strings.  It does produce a different sound for a while
but it is not what is ordinarily considered to be an on/off device,
But worst part of this is that anything pushed into the hammers tends
to be destructive.

I would absolutely not recommend the thumbtack method to anyone.  My
personal choice is the metal rod with the balls mounted on the ends of
the spring wires for obvious reason.

Hal Davis

 [ My early Seeburg piano has short copper cylinders suspended from
 [ rather stiff coated fabric strips.  The hammer strikes the strip
 [ above the metal cylinder, and the curved edge of the heavy cylinder
 [ strikes the unison strings.  It sounds very nice.  -- Robbie