Note: the font has to be loaded in plain TeX with
there is a bold extended slanted cm font, but only in 10pt --
basing the loading on what's in
plain.tex and the
tugboat style file, we get
\font\tenbs=cmbxsl10 \skewchar\tenbs='177 \font\twelvebs=cmbxsl12 scaled \magstep1 \skewchar\twelvebs='177
take a look at
tugboat.sty (included in tex live).
by the way,
\bs isn't a great name for this; it's often used as a synonym for
\font\bs=cs-lmbxo10 at 14 pt foo \bs bar \bye
It uses the latin modern, nearly the same as computer modern.