I've never seen the TA stem cable guide. Zip ties are lighter and cheaper for a weight weenie build but not really period correct,
So are there some threads on drilling holes on the stem split to relieve any potential stress risers? I haven't read about this. I have to admit, bar or stem failure on an old bike is something I think a little bit about. It's such a bad place for a parts failure. It's one of the reasons why I like running a new Nitto bar and stem on my builds.