3 1/2" X 3 3/4" (89mm X 95mm)

 (crown recess embossed) "FK inside G"

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Picture courtesy Bob Scafe

3 1/2" X 3 3/4" (89mm X 95mm)

 (crown inkstamp)

{Rosenthal} 

Picture courtesy Bob Berry

3 3/4" X 3 7/8" (93mm X 100mm)

 (crown inkstamp)

{stockhardt & schmidt} 

Picture courtesy Bob Berry

3 1/2" X 3 3/4" (89mm X 95mm)

 (crown incuse)

{Rheinisch-Westfalische Isolatorenwerke} 

Picture courtesy Bob Scafe