These instructions make clear that the Shimano LT-4 is a 4 speed system supporting a single chainwheel and a sprocket sets from 12-18 teeth to 12-21 teeth. This document is also usefully dated November 1988. Finally it is branded 'Shimano F1', with a funky, high speed, logo. I have yet to discover what this means.
And dig that crazy cable routing.
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Shimano LT-4 (LT45) 1988?