Depends on the cable guides; the oldest Mercian I've owned was a late 60s Campionissimo that had rear brake housing stops and derailleur housing stops at the base of the downtube and on the right chainstay. I believe the thinking was those would all work no matter what derailleur system one chose to use, or whether your shifters were on the downtube or the bar ends. I don't remember seeing brazed-on shifter bosses much until the late 70s/early 80s.

I do want to know more about later, though!