Removing Varnish

[1deu]

Working on a 12-A. Somewhere along the all the wood was heavily varnished...without removing the wood from the metal. Thanks to brush marks, drips and runs It needed doing.
Wood (I've left all honorable dings, scratches and dents), and metal has been done without damaging the patina. Problem is the butt plate. It too was varnished. In 1920 the plate was hard rubber. Which means that ordinary remover won't work; it'll eat the material. Anyone have any suggestions? Thanks again.