The forum cut was a GSC bagseed that got shared around the forums back when legit GSC was still hard to get.
OGKB is named for OGKushBreath, which is a guy’s forum handle. The story is he helped out someone from Cookie fam, took care of their grow for a while or something like that, and he got some GSC seeds in return. The OGKB cut came from one of those seeds. Is this story true? I dunno, but that’s the prevailing story.
The OGKB is a really distinctive cut. Crinkly leaves, duckfoot leaves, and it’s a notoriously slow vegger. You can find OGKB phenos in any GSC crosses though.
Both have produced great crosses. The FC is probably the cut most often used in GSC crosses. Nspecta said on his potcast interview that it’s his favorite of the GSC cuts. Dosidos, DVG lines, and Thug Pug lines are nearly all OGKB crosses.