This ECM is used in 1998 GM pickups, possibly others.
.ecu files describe the layout of tables and data items in a PROM image. There are several image editing programs that use .ecu files in conjunction with .bin files to allow you to edit the tables in a prom image in real world units rather than hex. Promedit and Winbin are two examples.
Prom images are listed by broadcast code, which uniquely identifies the application of the Prom (Engine, transmission, rear end, ECM, etc.)
Pictures of the circuit board: top, bottom
Service manual pages: Page 0, Page 1, Page 2, Page 3, Page 4, Page 5, Page 6, Page 7, Page 8, Page 9, Page a, Page b, Page c,