As far as I know, UA did NOT inherit the PRC library, and in any case, even if they did, MGM/UA would have these elements, not WB.

WB owns some of the Monogram library-- the deal was convoluted, and ultimately it only applies to the later years there.

If I'm not mistaken, Adrian Weiss bought at least part of PRC's library, in which case the elements (and not necessarily the rights, which were not renewed) are now held by Kit Parker Films. I know that this is the case with SCARED TO DEATH.