If it's pitch then it will melt with a hot air gun fairly easily. Any residue can be wiped up while still soft with a cloth to 'grab' it and the remainder will dissolve in acetone or white spirit,
With a lot of early electroforms, chased work and thin stampings it was very common to back them with with various types of pitch, rosin/resin and mastics to give them extra strength and weight as well as allow a flat rear surface if they were mounted.
Not uncommon to find leaded white metals used for the same reason.
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