White paint does appear post WWII, however;
Before removing the white paint I would do some research on the name and number painted inside (with what appears to be the same paint). It may well have been a re-issued WWII shell (during the Nam era) and that may well be Nam era graffiti? If by some luck you have helmet with some providence; A Vietnam era refurb would be quite easy (and easier on the wallet). The straps, cover and liner shouldn't be a problem to source.
Just a thought