Studying photos, it just seems that this American paint for undersides must be different in some way, in that it's not an exact match for proper British Sky Type S.  Am assuming these b/w period photos illustrate underside roundels painted over to adjust for smaller dimensions.   Though not huge, there is a noticeable difference between the undersides and this added paint:       Then there is some colour photos that indicate a blue hue to undersides of Corsairs, so