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".
In most other cases, Romulan warbirds were identified as "the Warbird T'Met", as noted in VOY: "Ship in a Bottle" and likewise in DS9: "The Die is Cast", or "the Romulan vessel Terix", as noted in TNG: "The Pegasus" and likewise DS9: "Paradise".