Tube Type Car Radios- What is a Spark Plate?

Car Radios in the past were designed to take a lot of mechanical stress and electrical stress. Another Item that needs to be added to the list is a static suppressor, consisting of a capacitor normally located inside the distributor. One side is connected to the mechanical distributor ground, and the lead is connected to the distributor point(s)on older cars.

 The spark plate itself was unknown to me when I first stared learning electronics.
Even, when the spark plate was in the schematic. I didn't know where the spark plate was located or even looked like

Then I had an Epiphany. HMM! Plate was a key. .
What was the purpose of that copper plate on the sidewall of the chassis?   detective mode engaged!

A spark plate is shown in a schematic where the "A" lead goes into a tube type car radio.
 It's symbol is that of a capacitor. It's purpose is to filter out some of the spark plug noise, and generator/alternator noise from the radio.

 But is sure doesn't look like a regular capacitor!   
 When I first tried to find the one in a radio I was repairing I didn't recognize what it was.   Then AHA! The brain cells finally got it.

  It will be a small square piece of copper sheet with insulation on the back glued to the chassis with the "A" lead going to it and another lead going directly to the power switch.(And maybe to the pilot light).

They are often damaged by someone carrying the radio by its power leads. They can totally rip off the side of the chassis.

I have had to replace a few over the decades. Copper sheet can be cut, fish paper insulation can be glued to the back, being sure the fish paper is larger than the copper sheet; then the fish paper glued to the chassis. Works fine as a replacement.
 Depending on how the Spark plate is ripped off the chassis, sometimes you can just re-glue it down. I like GC Plio-Bond cement. (Still available at Ace Hardware- Not a sponsor).

One can also just add a terminal strip and use a ceramic capacitor (50V or more) to replace the spark plate with. The spark plates came in different sizes also, so you either have to use the schematic to get the value (in PFD/ NFD) of it, or dig out the calculator and micrometer and determine the capacitance of the original one by measuring the thickness of the original insulator, the fish paper you have,  and the surface area of the original piece of copper sheet. (Or measure one of the same size in another radio).

 Hint: it won't be a very big capacitance value.