Look over at KHB Photographix
Kevin Brown's shop in Mississuaga stocks all sorts of enlarger bits. If it doesn't exist they will even make it.
Somewhere online on thier site there is a reference section of every Omega or Devere ever made. This should tell you about it.
I too have a B, and noticed the tapped hole, but have never bothered with a red filter so far. I was going to make one from a piece of clear acrylic, a bolt, a spacer, and red Rosco gel, but it is not a project that has worked up the priority list. I suppose that will have to wait for retirement. That for me is likely 20 years off, so the do it someday list will be very long by then.