The curved cut up on the backside of xylophone bars, orchestra bells,
marimba bars is there because that is the way they are tuned.

Art Reblitz has an excellent explanation of the process in his book,
"Rebuilding the Player Piano" but just in case you do not have the book
a brief explanation follows:

After the bars are cut to length, they pass to the tuner.  He has a
sanding drum in front of him, rotating very slowly with a rough grit of
sandpaper on it.  He grinds away the center portion of the bar to lower
the tone or note, checking his progress frequently with a strobe tuner.
The heat that builds up also affects the tone, so the bars must be
allowed to cool during the process.

If he grinds too much material off of the center of the bar, between
the nodes (that's the part of the bar that does not vibrate, where they
drill the mounting holes), he then grinds the ends of the bars to
correct himself.

Hope this helps.  Art's explanation is much better.

Ed Gaida