BowlerKid,
The coverstock on the Road Hawg and Pain are both Hammer's Juiced reactive coverstock, while they may have similar characteristics to GB, I wouldn't necessarily say they are variations on it. Though Ebonite does own Hammer, I'm under the impression that they do use different coverstocks, I wouldn't think there is much material sharing going on as they are, still technically two separate companies.
As for the Black Widow, I belive that one is the Violent Reactive coverstock, which I though was a new formulation for Hammer (with the HawgZilla having Ultra-Violent Reactive if i remember correctly...). I again wouldn't think there would be any relation to GB or GBV, though it would be easy to assume so.
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Andrew Loose
1/2 of ABT's Finest
"Strike for show, spare for dough, split for D'OH!"