3 1/8" X 4 1/4" (77mm X 104mm)

(skirt inkstamp) TTT

{maker unknown}

Picture courtesy Bob Scafe

3 1/4" X 4 1/4" (80mm X 104mm)

(skirt inkstamp) 1968 / BULGARIA

 OTE

{Product of Bulgaria}

Picture courtesy Bob Scafe

3 5/16" X 4 3/8" (82mm X 108mm)

(skirt inkstamp) OTE

{maker unknown}

Picture courtesy Bob Scafe

3 1/8" X 4 1/4" (77mm X 104mm)

(front skirt inkstamp)    OTE 

(rear skirt inkstamp)

                               NGK

                              MADE IN JAPAN

{NGK}

Picture courtesy Bob Berry

3 1/8" X 4 1/4" (77mm X 104mm)

        (dome inkstamp)         

(skirt inkstamp)    OTE 

{Product of Germany}

{KWH}

Picture courtesy Bob Scafe

3 1/8" X 4 1/4" (77mm X 104mm)

(skirt inkstamp)    T.T.T.

{maker unknown}

Picture courtesy Bob Berry