To raise the pitch on a xylophone or glockenspiel, remove material
equally from both ends.  To lower the pitch, remove material from the
back [underside], the centre part which has a curved profile.  Keep
this profile in its good curved shape whilst removing the material.

All work should be done with numerous checks.  There is a possibility
of some overtones which can oppose the basic pitch, and they can
confuse the person tuning and the tuning device to thinking the bar is
tuned too high or too low.  I had a similar condition when tuning one
of two glockenspiel bars a few years ago.

Stephen Bentley -  SB-"O" Rolls
Vancouver, British Columbia, Canada