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a prevalent little marking in many styles pops up frequently
To a large extent you answered your own question, although having an illustration or two would be helpful. The USPOD supplied steel duplex handstamps to 1st & 2nd class offices, if I recall. Many of fhe earlier devices had an attached oval killer of arcs and later, various barred ovals. Come contained numbers, letters, RMS, or other symbols or designations. Bottom line, they tended to be used in larger offices, thus more common. The vast majority will have modest value. This in contrast to the rubber handstamps such as Doanes or 4-bar handstamps issued to the lower class offices in the early 20th century. It all depends!