An early PX4, with sloping anode.
Another early PX4, see next picture.
Another early PX4, with normal anode.
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This PX4 introduced the waisted envelope -
The last pre-WWII PX4's had a finned anode
One of the last (1950's) PX4's
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They cost a little bit more nowadays! 
Now the PX4 has been remanufactured
Mullard made an alternative
