PWB was a three-letter acronym that was a common starship prefix in the Romulan Star Empire.
Vessels identified with this prefix included the PWB Aj'rmr, PWB Koderex, PWB Preceptor, PWB Tomal, and PWB T'Tpalok. (DS9: "Image in the Sand", "The Siege of AR-558")
The designation "PWB" appeared on casualty report in the above mentioned episodes, which assumes the otherwise unstated title "Praetorian WarBird"..
According to TNG
: "Face of the Enemy
", the Khazara was identified with the title "Imperial Romulan Warbird", which assumes the otherwise unstated prefix of "IRW".