It's not clear from the '91 Aluminum 354 catalog page that the rear brake cable has internal routing, only that it doesn't have the top tube cable guides.
However, the 1991 High Plains Aluminum MTB catalog page makes a point of hyping the internal cable routing on that model as "...a feature found only on expensive, custom mountain bikes", so clearly some early nineties Schwinn aluminum frames had internal routing for the rear brake cable.
