This information can be found in the FBI website in their vault section. It's from 1974 and stated that some of BGF members were indeed black panthers. You can't overlook the fact that George Jackson was a member and panthers that were arrested/in prison during those times would join BGF ( not all of course but some).
George Jackson was named Field Marshall of the Black Panther Party by Huey P Newton. Yes, some members of the Panther Party mostly the Panther underground became part of the BGF. However one must remember that many believed Huey betrayed Jonathon Jackson when he took the hostages and was subsequently killed. There is some not so friendly feelings that many of the old BGF have/had toward Huey because of this.
