3 7/8" X 3 7/8" (97mm X 97mm)

 (dome inkstamp) RIG

{Rosenthal Isolatoren GmbH}

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Picture courtesy Bob Berry

3 3/4" X 3 7/8" (95mm X 97mm)

(top ink stamp)

{Rosenthal}

Picture courtesy Bob Berry

3 3/4" X 3 3/4" (95mm X 94mm)

(top ink stamp)

{Stemag}

Picture courtesy Bob Berry

3 7/8" X 3 7/8" (96mm X 96mm)

 No markings

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Picture courtesy Bob Berry