[aerotruk63]

So far that`s the information that we`ve found. The two letters can change with every production year. The RPO `Regular Production Order indicated what was asked for and the two letters informed the guy on the assembly line to put the correct matching parts. For instance a 292 motor for a 4 speed had different ECL than a Powerglide . The assembly line must have had engines set up with clutch assemblies as well as flexplates for Automatics. I still guessing at this point.