What was found
AFTER TAKEOFF AT 1400 FEET THE RT ENGINE THRUST REVERSER DEPLOYED. THE ENGINE WAS SHUT DOWN AND FLIGHT RETURNED TO MKE. NO CHECK-C. *S/D* INVESTIGATION FOUND THE REVERSER UNLOCK LIGHT WAS ON AT INITIAL DEPARTURE AND CREW RETURNED TO THE GATE. MAINTENANCE DEACTIVATED THE REVERSER ELECTRICALLY AND HYDRAULICALLY BUT MISUNDERSTOOD THE INSTRUCTIONS CONCERNING INSTALLATION OF LOCKOUT PINS, AND DID NOT INSTALL THEM. THAT ENABLED THE REVERSER TO DEPLOY IN FLIGHT. THE INSTRUCTIONS HAVE SINCE BEEN CLARIFIED, AND DISPATCH WITH THE UNLOCK LIGHT ILLUMINATED IS NOW PROHIBITED.