Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
You are actually confusing the actual component code which is LM139 with the component identifier on the schematic.
U1,U2,U3 etc represent one chip each and the letters A,B,C,D represent the four comparators included in each chip.