Wow, we’re going way back if we’re talking about crystals. Anyway, let me go ahead and share based on my recollection of things back in the day.

Generally, the frequency designation on the receiver will indicate the range that it is meant to work on. Here in the US, Surface stuck to 72 and 75MHz, with older radios running on 27MHz. Obviously the radio and receiver will be paired with a set of crystals, which should be on the same channel. It’s important to note however, that back then, crystals were usually matched from the manufacturer and available in pairs for the radio and receiver. The Tx crystal in the radio, the Rx crystal in the receiver.

This is because even though they are working on the same channel, crystals might be slightly off from each other if you just grab any pair. As that was the case, crystals were matched from the manufacturer to minimize any glitches.

Another important thing to note is that the radio system could have been either AM or FM; FM being the later technology, which is around the time when PCM technology was just starting to come in to play. These are also dependent on the radio, so you want to make sure that you get AM crystals for AM radios, and FM crystals for FM radios. Finally, you’ll want to stick with crystals from the manufacturer of the radio, as again, these were tuned for use with those systems.