The more I look at this, the more I'm convinced that this is a inept fiberglass fan-copy (with clumsy alterations) made from a
Rubies/Don Post Deluxe Vader to try and make an RotS Vader, hence the front of the brow ridge on the dome being cut flat and the all black paint scheme. This screams to me of an unfortunate eBay purchase originally from around that time.