Many of the Korean War Mustangs that were collected from various units and went through major overhaul at the Sacramento and San Antonio Air Depots before being shipped to Japan likely had the original surfacer/putty removed and then were painted in silver lacquer. I have read that the Mustangs that were held in storage in Japan after WW2 were also put into use, so were more than likely to still have the factory applied finish ion the wings- I am guessing that they were mostly low-time airplanes and so chances were good that they were still in their original finish, where the wings were concerned. You can see, in good quality photos of Korean War Mustangs, that some are still in natural metal with painted filled wings, and some have stripped wings and are painted overall with aluminum lacquer..
Restored Mustangs on the civil register can be seen both with and without the surfacer/putty/aluminum paint on the wings, and some can be seen with stripped wings in either painted or polished form. There is not much need for a Mustang's wings to be in the original finish to maintain a perfect laminar flown nowadays, as maximum combat performance is no longer required, and the ,low octane/low lead avgas that is available saps a lot more performance than not having a filled, puttied, and painted wing.
Mike