Why, a receiver, will you call RX and a transmitter TX?
RX is Receiver, and TX is a mark of Transmitter.
Because there is also something to correct and use free-running oscillator besides the crystal control for a receiver and transmitters and others of course, if X is an abbreviation of X’tal = Crystal, unfortunately the opinion I say is a slip.
Simply, X is the character X which indicates abbreviation. In other words, when it’s original, initial R and below that omit Receiver, Transmitter and the place which should be transcribed in X, and it’s said that they make the RX TX likewise by T+X.
Transceiver, TRX or XCVR, Receiver omits RCVR and Transmitter with XMTR again.
A receiver calls Receiver (receiver) and a transmitter Transmitter (transmitter) in English. The X which indicates a crystal oscillator necessary to the first letter and receiver-transmitter (Crystal=X’tal) is combined, and they’re RX and the one reduced with TX.
By the way, a word as the transceiver which indicates a walkie-talkie is the coined word with which a transmitter and a receiver were combined literally. A shortened form is called TRX of course.