If there is one place where someone is sure to know... this is it!

It pertains to the Rx. It has nothing to do with analog/digital. Single conversion crystals won't work with dual conversion receivers and vice versa. ass_u_me... bad new bears... as I recall

Make sure you get a crystal that matches your receiver!

Here is my favorite technical description:
http://www.torreypinesgulls.org/Radios.htm

It has to do with the number of stages of demodulation that are going on in the input stage of the receiver before the signal is amplified. Single conversion has a single stage. Dual has two. It is only an issue with FM receivers. Dual conversion is a way get around some niggly technical problems that can lead to "cross talk" (for lack of better term) between certain channel pairs. The torrey pines site describes it far better than I every will.

What kind of receiver do you have?