There are many good designs for the "mandolin" or "banjo" or "harp" attachment which gives the piano a "honky-tonk" sound.  For the same hammer velocity, the basic design shown in the three photos below gives the most sound volume, compared to the ordinary piano volume, due to the mass of the cylindrical metal piece which strikes the strings.  Other designs use thin metal tabs or a wire coil to strike the strings, or wooden pegs, as described below by B Bronson.
 

Typical appearance in a vertical piano, shown here in the Fratihymnia orchestrion being restored by Willy van der Reijden, Almelo, The Netherlands.  Willy says the Frati company called it the "harp effect".  The supporting fabric and the suspended metal piece are aligned with the face of the hammer (not aligned with the strings).  Horizontal alignment is critical, else two notes may sound together!  The hammer face strikes the thin supporting strap just above the metal piece.

 

Back side (nearest the strings) of the "mandolin" device in an early Seeburg nickelodeon.  The copper tube is 0.125" (3.2 mm)  i.d., 0.188" (4.8 mm) o.d., 0.425" (10.8 mm) long, and the attachment loop is 0.038" (1 mm) o.d. copper wire.  Scraps of copper and steel wire fill the inside, for added weight.  (A lead alloy, such as solder, might also be used to fill the tube.)

The support strap is imitation leather, thickness 0.015" (0.4 mm).

 

(Photo by B Bronson) This is very familiar in the full size Coinola nickelodeon.  I don't know if it was used in the Midget series  It  is actually a wooden slat drilled precisely through the thin dimension so that there is a guide hole for the notes on the treble end.  A metal rod goes through the hole with a wooden button each end. The system is installed between the hammers and the strings and has slotted holes in the slat which fit over pegs that allow it to move left and right.  When the music roll calls for the mandolin effect, a pneumatic pulls the system to the right, placing the back end of each rod in front of the hammer shank.  As the note plays, the shank hits the rod, throwing it at the string, which is hit with the front button.  The rod is returned by a light coil spring. With the mandolin "off", the hammer shanks fly up between the rods.  The ones I have seen have been engineered to require the rod to hit the string from momentum.  In other words, if one gently pushed the hammer fully, the rod would not be touching the string. Another variation on the hanging strip theme was that used in the Nelson- Wiggens.  I have seen a couple that appeared to be quite original which used a pump type cloth with a thin (1/8") wood piece glued on each strip.  The few I have seen appeared to have the wood directly between the hammer and string. More Coinola photos at mandolin2.html. B Bronson Fri, 7 Jan 2000 14:41:09 -0500

The Marquette Cremona usually had this plunger style mandolin device.  The thin wood wedges resemble a tiny paddle for handling pizza in a hot oven!  A thin piece of leather covers the end of the plunger stick, which is struck by the hammer shank.  A tiny coil spring returns the plunger to its rest position. When it's properly adjusted the hammer shank does not force the plunger into the strings; rather, the hammer shank propels the plunger like a cue stick hitting a billiard ball.  The plunger actually strikes the strings and rebounds before the hammer hits the strings. The ends of the paddles shown in the photo are quite worn because the hammers became worn and were filed, so the hammer shank actually forced the paddle into the strings.  This also caused rapid deterioration of the leather.

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