The response I received from the Hi Viz product manager was, "The Ruger HRB1001 front sight will fit all Ruger bull barrels," so you're in good shape.
Curiously, I just found Williams front and back FireSights available at The Gun Source (www.thegunsource.com) though I thought Brownells said...
Thanks for the tip; I called Hi Viz, and they said their Mark II front sight will fit the Mark I. For the rear sights, I'm going to try a paint-on by Bright Sights, just need to pick a color.
My 51-year-old eyes can no longer focus on the adjustable factory sights on my venerable Mark I A100 Bull-barrel. Anyone know of any vendors out there making glow or fiber optic sights for the Mark I (or for Mark II/III that will fit Mark I)? Similar to Williams FireSights (which I learned...