The Pen20 was issued in 1933. The Mullard box design was changed in the mid 1930s making this an early box for the Pen20. This carton is top opening with a tuck-in flaps to hold the valve below the top of the package. Inside the box is a shaped card liner to hold the valve. The box is 70 mm square and 148 mm tall.
 
 
 
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