I recall reading that they were meant for the epiphone line, but I cannot find a reference in the book I thought I read it so don't take my word for it. Seems likely, after all they made the mini humbucker small just to have something different on the epi guitars - but it ended up on many Gibsons too. The metal cover's on the 330 was introduced about the same time (1962) as the model was 'upgraded' to block inlays.
Interesing too, that the first P-90 type pickup that came in 1940 was metal covered. The version we all know and love came around 1946 with plastic cover.