The "21" likely refers to the form factor (introduced in 2001 as "mPGA478B"). Boards bearing this socket accommodated Pentium 4 and Celeron processors on a 400/533/800 MHz front-side bus. The "B6" fragment might be an internal revision notation for an Intel Desktop Board like the D845GB, D845PE, or D865PERL. These boards featured:
It included a PCIe 3.0 x16 slot for dedicated graphics. Intel Desktop Board 21 B6 E1 E2 Er