I had endless problems with the flat leather belt drive on my
Wurlitzer 125 band organ.  The leather kept stretching and I was
constantly taking the belt to the local harness maker to have it
shortened and re-stitched.

My permanent solution was to change from leather to a rubber/fabric
material, black in colour, available at the Tractor Supply Company
(TSC Stores).  It is sold as "baler belting" and can be cut to any
length or width.  I had it "laced" with the metal joint that can be
easily taken apart if required.

This belt has never slipped or stretched.  I operates well in most
any temperature.  It has been in service now for twelve years and
shows no sign of failing.

Hope this may solve your problem.

Ken Vinen
Aylmer, Ontario, Canada