Bruce From United States of America, joined May 1999, 5068 posts, RR: 15 Posted (12 years 4 months 1 week 1 day 4 hours ago) and read 2276 times:
I have seen this on most of my flight deck visits and all the photos I see. There is the plane's registration # located on the instrument panel, and near it there is another code. usually 2 letters dash 2 more letters. Check out this photo:
Backfire From Germany, joined Oct 2006, 0 posts, RR: 0
Reply 3, posted (12 years 4 months 1 week 1 day 4 hours ago) and read 2244 times:
The four characters are the Selective Calling or SELCAL identifier which is used to establish initial contact on HF radio frequencies. The four characters precedes voice communication and provides identification of the aircraft involved - although the fact that there are more aircraft than possible SELCAL combinations means that there still has to be positive confirmation once contact is established.