If it’s a thin newspaper printing plate from that era, it’s likely aluminum, and the white spots may be surface oxidation. Start very cautiously: test any cleaning only on a small, non-image or non-critical area first.

One suggested approach is to make a mild paste of cream of tartar with a little warm water, then gently rub the test area with a clean cotton cloth and rinse. If that works without harming the surface, you could proceed carefully on other affected areas.

Avoid anything abrasive, since printing plates can be easily scratched and the image detail may be delicate. Commercial aluminum cleaners exist, but their abrasiveness is uncertain, so they’re risky unless known to be very gentle.

Because this is a family keepsake, the safest option is to proceed minimally and stop if the surface changes unexpectedly. If the plate is historically or sentimentally important, consider consulting a conservator before doing more aggressive cleaning.