Every Nelson-Wiggen Style 5x I've restored had a 20-note marimba
section, playing unison with the piano notes.  The 28-note xylophone in
the 4X plays an octave higher than the corresponding piano note (as all
coin piano xylophones do).  A "xylophone" becomes a "marimba" basically
by adding tuned resonators to each bar, like the Cremona J has
protruding back behind its case.

S. K. Goodman