RTB is a Road
All Terrain Bike. I first came across the term in C+, in the days before they'd discovered road bikes & sportives etc.
But the first hybrid bikes - the Orbit Frontier (not that one - the earlier one), and the Muddy Fox Cantremember - were essentially ATBs with drop bars.
When I went to the Bicycle Action ride at Box Hill in about 88?, the couriers were there, with their essential mountain bikes - trendy, but necessary for those city streets, d'ye see? (how times change!
). Several of them had unusual bars, including the Bramahs, and drop bars with extended straights forming grips each side. One had straight bars with bolted in drop extensions, an idea which has been revived recently, but more successfully. Having a tourer which loved roughstuff (my old Carlton at the time), I was quite interested in this convergence.