[Milsurplus] Crystal Curiosity

[Tom Brent]

I was sorting through a pile of recently acquired quartz crystals today (I
know, I need to get a life) and came across four in HC-6/U cases, all made
by a firm named Hill in 1967 with the following frequencies:

497.5 KHz
498.5 KHz
501.5 KHz
502.5 KHz

I presume these are for something operating with a 500 KHz center
frequency, can anyone tell me what equipment it might be?

Happy tinkering.

Tom
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