In This is Spinal Tap, Rob Reiner plays a documentary filmmaker named Marty DiBergi who is wearing the same hat throughout the entire film which looks like its meant to be some sort of joke due to its alteration from the real version of the same hat.
His hat says "USS OORAL SEA OV-4B" while the real hat should say "USS CORAL SEA OV-43"
So what exactly is the joke here?