Hah, in California I used to have HX.

The short plate came back to bite me: Years after I had moved to another state, an automatic license plate reader on a toll road (91 Express Lanes) in Los Angeles misread someone else's plate as mine. It was kafkaesque: My public records request for photographic evidence was blocked because, if I was correct that the offending driver was not me, the law prohibits the release of records revealing others' driving patterns.

The other plates available when I did a similar search were BO and IR. In retrospect IR wasn't a bad choice.