The production people on PoC had enormous issues with the vinyl squadron codes peeling off in flight.

For the bulk of the filming they were all removed and a little bit of dialogue written in for S/L Rex after the Blenheim incident saying “I’ve had all the squadron codes removed.. in case it happens again” (That’s paraphrased, not verbatim) to explain their absence.

Moggy

Even so, there are a couple of brief shots later on in the series with one Spit (possibly a replica) clearly showing the “NS” codes…