Help id mystery capacitor

I got these assisting clearing out a storage. There was lots of military parts as well as all kinds of electronics. Anyways these caps are a bit mysterious. They measure dead flat up to tens of MHz.
I took one pack apart and there are two caps a little fatter than the last one. The bigger ones measure a tad over 400nF.
Are they mica?
Guess I will need to build a new phono pre or something to try these out.
Thanks for help. I have tried googling and no luck.
 

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I doubt they can be mica. Mica is a crystaline material, not noted for being able to bend, nor for being available in long rolls given its mined, not manufactured.
The construction looks like flat-rolled film construction, like LiPo batteries, used for its spatial efficiency compared to cylindrical roll.
 
a measuring instrument's oscillator or time base would be a more appropriate use than a phono preamp .There's no vinyl to play 10 mhz ...

I used an HP analyzer, the phono pre is just an idea to use 400nF-ish capacitors.
The ability for a cap to be flat up into MHz is just a sign it is non inductive probably mute for audio but in a way a quality figure.