It's been mentioned that perhaps this adhesive is the wax used
to attach leather valves for reeds in an accordion.  I've rebuilt
a number of accordions, and the wax used is a compound of beeswax
and rosin.

Here is a source of this wax, and it is the same wax used in
Castelfidardo, Italy, the "home" of most quality accordions
made today:

https://frment.com/reeds/accordion-reed-wax/ 

The wax is applied using heat, and I use a low power soldering
iron, with a modified tip to fit between the reeds.

John Bartlett
Woodruff, South Carolina