Although 2902s and 2903s were more than likely factory-painted for custom order. It is also possible that they used actual white plastic in other cases.
The “White” Simplex 2902-9735s are “factory-painted” and the paint job is very precise and well-done.
However, the 2902-9734, which was most likely produced at the same time, came in white by default. What’s more, it uses actual white plastic, and anyone could tell it’s real plastic by noticing the colour yellowing caused by UV exposure.
Let me know what you think. I personally think that both are plausible enough.