I've done a number of soaks with 1950s cards, almost always as a cleaning method to remove scrapbook paste and improve readability. Whether you should do it depends on the desired outcome. Do you want to remove something stuck on the front or back? Make it pliable and iron out card wrinkles? (To temper expectations, soaking almost never improves a grade, as the drying process can introduce warping or remove color.)
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