The Tektro RX 4.1 "Reverse" Lever works with caliper or traditional cantilever brakes and include external cable routing.
All reverse levers work with caliper or cantilever brakes
All reverse levers have external cable routing
Fits bar I.D. of 19-21mm
These attach by expander plugs into the ends of the handlebars. They work with normal-travel calipers or traditional center-pull cantilevers, not with "V Brakes" or other "direct pull" cantilevers.
These work with pretty much any type bullhorn handlebar. They will not fit in most mountain bike, or North Road handlebars.
In most applications, they put the part of the lever with the greatest leverage under you strongest, longest fingers, so they have real ergonomic benefits.