Judging by the description, I'll bet its topology is similar to the Langevin EQ251A. See this page. But this looks to be a bit more sophisticated since it gives reciprocal curves, which the Langevin did not.
The "minimum loss" two-band type using a common bridged-T section is credited to Art Davis, who founded the Cinema Engineering company.
I designed a passive EQ that can replicate the curves of the 251A but does not possess its "constant impedance" advantage. However, it's a simpler circuit that uses readily-available parts. Here's one of a few threads on this board that discuss it: http://www.groupdiy.com/index.php?topic=4107