Stripped thread for BB cable guide

+1 for the M6 bolt, done it a few times before and had no problems

You don't need a guide. Just run the bare cable under the BB and up between the chainstays. Nothing to clog up - or you could fit a short length of outer cable if you're worried about the paintwork. In my experience very little wear occurs.

I would have thought that an expensive frame like a Master would have had the guides brazed on the BB

Older versions of the Master did, but I would guess this ended in the late 80's/early 90's. I actually prefer a replaceable guide. It's kinder on the paintwork, plus I get the feeling that a plastic guide makes for slightly smoother shifting than metal-on-metal (If I hadn't spent way too much money on the Campag "Maximum Smoothness" derailleur cable set I probably wouldn't care very much ).