Hello Kristine,
I'm pretty sure that Culver Ltd didn't manufacture any glass themselves, but were only a decorating firm, who bought their blanks from a number of different glass companies, then decorated and sold them under their own name is all, which is likely why there is so little information to be found on them when looking for them as a glass company.
There were many decorating firms like this, Dorothy Thorpe is another name and I believe Georges Briard also used some other glass companies blanks, but did make all of the slumped glass items they decorated and sold.
Mike